CI: switch to GitHub Actions

This commit:
* Adds a GH Actions workflow for the CI checks which were previously run on Travis.
* Removes the, now redundant, `.travis.yml` configuration.
* Updates the `.gitattributes` file.
* Updates the "Build Status" badge in the Readme to use the results from the GH Actions runs.

Notes:
1. Builds will run on all pushes and on pull requests. Builds can also be manually triggered.
    Note: manual triggering of builds has to be [explicitly allowed](https://github.blog/changelog/2020-07-06-github-actions-manual-triggers-with-workflow_dispatch/). This is not a feature which is enabled by default.
2. If a previous GH actions run for the same branch hadn't finished yet when the same branch is pushed again, the previous run will be cancelled.
    In Travis, this was an option on the "Settings" page - "Auto cancellation" -, which was turned on for most, if not all, repos. The `concurrency` configuration in the GHA script emulates the same behaviour.
3. The default ini settings used by the `setup-php` action are based on the `php.ini-production` configuration.
    This means that `error_reporting=E_ALL & ~E_DEPRECATED & ~E_STRICT`, `display_errors` is set to `Off` and `zend.assertions` is set to `-1` (= do not compile).
    For the purposes of CI, especially for linting and testing code, I'd recommend running with `zend.assertions=-1, `error_reporting=-1` and `display_errors=On` to ensure **all** PHP notices are shown.
    Note: the defaults will be changed in the next major of `setup-php` to be based on the `php.ini-develop` configuration, but that may still be a while off.
    Refs:
    * shivammathur/setup-php#450
    * shivammathur/setup-php#469
4. Composer dependency downloads will be cached for faster builds using a [predefined GH action](https://github.com/marketplace/actions/install-composer-dependencies) specifically created for this purpose.
    The alternative would be to handle the caching manually, which would add three extra steps to the script.
    Note: Caching works differently between Travis and GH Actions.
    On GH Actions, once a cache has been created, it can't be updated. It can only be replaced by a new cache with a different key.
    As the PHP version, the `composer.json` and a potential `composer.lock` hash are all part of the key used by the above mentioned action, this difference should not have a significant impact.
    Ref: https://docs.github.com/en/actions/advanced-guides/caching-dependencies-to-speed-up-workflows

Includes adding new builds against PHP 8.0 and 8.1.

Author: jrfnl

.gitattributes

@@ -1,6 +1,6 @@
 /.gitattributes export-ignore
 /.gitignore export-ignore
-/.travis.yml export-ignore
+/.github/ export-ignore
 /CONTRIBUTING.md export-ignore
 /fixtures/ export-ignore
 /phpunit.xml.dist export-ignore

.github/workflows/ci.yml

@@ -0,0 +1,61 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+  # Allow manually triggering the workflow.
+  workflow_dispatch:
+
+# Cancels all previous workflow runs for the same branch that have not yet completed.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  check_composer:
+    name: Check composer.json
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.0'
+          coverage: none
+
+      - name: Validate composer.json
+        run: composer validate --strict --no-check-lock
+
+  test:
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        php: ['7.3', '7.4', '8.0', '8.1']
+
+    name: PHP ${{ matrix.php }}
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          ini-values: zend.assertions=1, error_reporting=-1, display_errors=On
+          coverage: xdebug
+
+      - name: Validate Composer installation
+        run: composer validate --no-check-all --strict
+
+      # Install dependencies and handle caching in one go.
+      # @link https://github.com/marketplace/actions/install-composer-dependencies
+      - name: Install Composer dependencies
+        uses: "ramsey/composer-install@v1"
+
+      - name: Execute Unit Tests
+        run: vendor/bin/phpunit --coverage-text

.travis.yml

@@ -1,6 +1,6 @@
 # Prophecy
 
-[![Build Status](https://travis-ci.org/phpspec/prophecy-phpunit.png?branch=master)](https://travis-ci.org/phpspec/prophecy-phpunit)
+[![Build Status](https://github.com/phpspec/prophecy-phpunit/actions/workflows/test.yml/badge.svg)](https://github.com/phpspec/prophecy-phpunit/actions/workflows/test.yml)
 
 Prophecy PhpUnit integrates the [Prophecy](https://github.com/phpspec/prophecy) mocking
 library with [PHPUnit](https://phpunit.de) to provide an easier mocking in your testsuite.