Update CI to use official Swift 5.2 images and update README for current testing matrix

Author: Mordil

.gitlab/ci/ubuntu-bionic.yml

@@ -12,7 +12,7 @@ swift 5.1:
 
 swift 5.2:
   extends: .unit-test
-  image: swiftlang/swift:nightly-5.2-bionic
+  image: swift:5.2-bionic
 
 swift trunk:
   extends: .unit-test

.gitlab/ci/ubuntu-xenial.yml

@@ -12,7 +12,7 @@ swift 5.1:
 
 swift 5.2:
   extends: .unit-test
-  image: swiftlang/swift:nightly-5.2-xenial
+  image: swift:5.2-xenial
 
 swift trunk:
   extends: .unit-test

README.md

@@ -111,7 +111,7 @@ receive regular unit testing (either in development, or with CI).
 
 | Platform | Swift 5 | 5.1 | 5.2 | Trunk |
 |:---------|:-------:|:---:|:---:|:-----:|
-| macOS Latest | | :white_check_mark: | | |
+| macOS Latest | | | :white_check_mark: | |
 | Ubuntu 18.04 (Bionic) | :white_check_mark: | :white_check_mark: | :white_check_mark: | :white_check_mark: |
 | Ubuntu 16.04 (Xenial) | :white_check_mark: | :white_check_mark: | :white_check_mark: | :white_check_mark: |