Travis-ci: remove unsupported rubies and rails versions

[skatkov]

Removing some unsupported stuff from travis ci matrix - rubies and rails.

Based on a comment shared by @schneems in #468 (comment)

[schneems]

I don't think we can kill off 5.1 support just yet until Rails 6.1 comes out.

I'm also not totally sure what repercussions this will have in terms of maintenance policy. Basically: rails versions don't declare a dependency on sprockets, they declare it on sprockets-rails. I would normally say that if we're dropping support, we should consider branching this gem so 4.x would be the version that has support dropped for these older rubies and rails versions, and 3.x could possibly live around as a best-effort, minimially supported thing if the process causes havoc and/or other core memebers need something in an older version.

[rafaelfranca]

I don't think we can kill off 5.1 support just yet until Rails 6.1 comes out.

Yes, we can. Only Rails 5.2 and 6.0 are currently supported.

I don't think we need to release a major version of sprockets-rails just to drop support to old Rails version. We only need to update the sprockets-rails.gemspec to not allow it being installed in any Rails < 5.2.

[skatkov]

Agree with @rafaelfranca. Not sure why we can't drop 5.1, since only 5.2 and 6.0 are supported now.

I also updated gemspec.

[schneems]

Only Rails 5.2 and 6.0 are currently supported.

Right, sorry. I got the Rails support policy (2 releases) and Ruby (3 releases) mixed up in my head. Thanks for the clarification. I've been away from github.com/rails for too long 😅

Thanks for the work here!