Support Rails 7 alpha

[SeanLF]

closes #94

Note: rails 7 requires ruby 2.7 minimum, not sure how to modify the test matrix to only test rails 7 with ruby 2.7 and later

Changes

• adds ruby 3 to the test matrix
• adds support for rails 7 alpha

Tests

[Mange]

This looks excellent. Thank you for taking time to add this! :heart:

Note: rails 7 requires ruby 2.7 minimum, not sure how to modify the test matrix to only test rails 7 with ruby 2.7 and later

I have an idea and will experiment a bit with it.

[Mange]

I've squashed together your changes into a new commit and opened a new PR with the Travis changes in #97.

[SeanLF]

Thanks @Mange!

[Mange]

Version 2.3.0 was just pushed with this included.