Drop support for End-of-Lifed Rails versions

[seanpdoyle]

At the time of pull request submission, official Rails support for versions 4.2, 5.0, and 5.1 have ended

This commit configures the continuous integration test suite to execute against 5.2 and 6.0.

To support those versions in our continuous integration containers, remove configuration code to limit the bundler version to < 2.0.

Additionally, this commit amends the README to mention 5.2 as the minimally supported version.