Update contributte/phpstan requirement from ~0.2.0 to ~0.3.1 (#105)

Updates the requirements on [contributte/phpstan](https://github.com/contributte/phpstan) to permit the latest version.
- [Release notes](https://github.com/contributte/phpstan/releases)
- [Commits](contributte/phpstan@v0.2.0...v0.3.1)

---
updated-dependencies:
- dependency-name: contributte/phpstan
  dependency-version: 0.3.1
  dependency-type: direct:development
...

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Author: dependabot[bot]

composer.json

@@ -15,7 +15,7 @@
     "thepay/api-client": "^2.0.0"
   },
   "require-dev": {
-    "contributte/phpstan": "~0.2.0",
+    "contributte/phpstan": "~0.3.1",
     "contributte/qa": "~0.4.0",
     "contributte/tester": "~0.4.1"
   },