Bump supported ruby versions

[jdleesmiller]

Upgrade minimum supported ruby versions to include only supported rubies. This is planned as part of an upcoming major release (2.0).

This will hopefully make CI less fragile, allow us to upgrade our linting system, and allow us to take advantage of newer ruby features.

Fixes #404 .

• Ruby 2.3 has been EOL since the end of March 2019.
• Support for JRuby 9.1 ended at the end of 2017.
• JDK7 is still in extended support, but it seems that JRuby has dropped it from CI, and it has caused problems recently (Fix CI and try rbx-4 #399).
• It would be nice to move from trusty to xenial, but that currently prevents testing on rbx (Fix CI and try rbx-4 #399). The canary rbx-4 build still seems to be broken. To be fair, the tests fail on rbx, and we don't fail the build, so we could give up on this and move to trusty now, but so far it doesn't seem to cause other problems.

Users who need older versions of ruby will still be able to use the planned 1.3 release for now.

I have tried to update the README and gemspec to make this clearer (and will update the changelog as part of the next release), to avoid confusion like in #254 .

Edit: I will leave this open for approximately one week for comment.

CC @hainesr @simonoff

[hainesr]

Could we leave a couple of EOL rubies in the "allow failures" section of the CI?

Also, this PR will bump the rubyzip version to 2.0.0, per Semantic Versioning, so should this be included too? Or is it OK to do separately?

Otherwise, great.

[jdleesmiller]

Thanks for the feedback!

Could we leave a couple of EOL rubies in the "allow failures" section of the CI?

I like that idea, but it seems that setting the 2.4 version constraint in the gemspec prevents it from running on older rubies:

    rubyzip was resolved to 1.2.4, which depends on
      ruby (>= 2.4)

I don't know of any straightforward way around that.

Or is it OK to do separately?

I'm planning to bump the version number separately, after merging the outstanding PRs. I think that will be OK.

[hainesr]

Good point about the versions - I obviously had my brain turned off at that point!

Yes, the only solution is to not test the older ones. That's fine. 👍