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.