ci: Update to Swift 4.2.3, use official Docker images (#38)

Author: djones6

.swift-version

@@ -1 +1 @@
-4.2.1
+4.2.3

.travis.yml

@@ -12,28 +12,27 @@ branches:
 matrix:
   include:
     - os: linux
-      dist: trusty
+      dist: xenial
       sudo: required
-      env: SWIFT_SNAPSHOT=4.0.3
-    - os: linux
-      dist: trusty
-      sudo: required
-      env: SWIFT_SNAPSHOT=4.1.3
+      services: docker
+      env:  DOCKER_IMAGE=swift:4.0.3 SWIFT_SNAPSHOT=4.0.3
     - os: linux
-      dist: trusty
+      dist: xenial
       sudo: required
+      services: docker
+      env: DOCKER_IMAGE=swift:4.1.3 SWIFT_SNAPSHOT=4.1.3
     - os: linux
-      dist: trusty
+      dist: xenial
       sudo: required
       services: docker
-      env: DOCKER_IMAGE=ubuntu:16.04
+      env: DOCKER_IMAGE=swift:4.2.3
     - os: linux
-      dist: trusty
+      dist: xenial
       sudo: required
       services: docker
-      env: DOCKER_IMAGE=ubuntu:16.04 SWIFT_SNAPSHOT=$SWIFT_DEVELOPMENT_SNAPSHOT
+      env: DOCKER_IMAGE=swift:4.2.3 SWIFT_SNAPSHOT=$SWIFT_DEVELOPMENT_SNAPSHOT
     - os: linux
-      dist: trusty
+      dist: xenial
       sudo: required
       services: docker
       env: DOCKER_IMAGE=ubuntu:18.04