Update Test Matrix to include Swift 5.1 Official Release

Motivation:

With the release of Swift 5.1, the official docker containers are available for both 16.04 and 18.04 of Linux - so the test matrix should include them.

Modifications:

- Add new 5.1 jobs for 18.04 linux and change 16.04 5.1 jobs to use the official docker images.
- Update the README to capture the changes

Result:

The number of configurations the library is tested against should be as wide as possible for regular interval testing passes.

.gitlab-ci.yml

@@ -47,7 +47,7 @@ build 5.0:ubuntu-xenial:
   image: swift:5.0-xenial
 build 5.1:ubuntu-xenial:
   extends: .build
-  image: norionomura/swift:swift-5.1-branch
+  image: swift:5.1-xenial
 build latest:ubuntu-xenial:
   extends: .build
   image: norionomura/swift:nightly
@@ -56,6 +56,9 @@ build latest:ubuntu-xenial:
 build 5.0:ubuntu-bionic:
   extends: .build
   image: swift:5.0-bionic
+build 5.1ubuntu-bionic:
+  extends: .build
+  image: swift:5.1-bionic
 
 .test:
   stage: Test
@@ -77,7 +80,7 @@ test 5.0:ubuntu-xenial:
   image: swift:5.0-xenial
 test 5.1:ubuntu-xenial:
   extends: .test
-  image: norionomura/swift:swift-5.1-branch
+  image: swift:5.1-xenial
 test latest:ubuntu-xenial:
   extends: .test
   image: norionomura/swift:nightly
@@ -86,3 +89,6 @@ test latest:ubuntu-xenial:
 test 5.0:ubuntu-bionic:
   extends: .test
   image: swift:5.0-bionic
+test 5.1:ubuntu-bionic:
+  extends: .test
+  image: swift:5.1-bionic

README.md

@@ -103,7 +103,7 @@ receive regular unit testing (either in development, or with CI).
 | Swift Version | macOS Mojave | Ubuntu 16.04 (Xenial) | Ubuntu 18.04 (Bionic) |
 |---|:---:|:---:|:---:|
 | 5.0 | X | X | X |
-| 5.1 | X | X |  |
+| 5.1 | X | X | X |
 | Trunk | X | X | |
 
 ## License