Update CI settings and fix build issues

[dlackty]

• [x] Remove Ruby 2.2 & 2.3 as they've been deprecated for 1+ year.
• [x] Add Ruby 2.7 to test target
• [x] Test Rails 6
• [x] Add Sprocket's new requirement manifest.js
• [x] Cache bundler in Travis CI
• [x] Fix up builds

[zorab47]

Nice! I'm way behind on open source libs. ❤️