Drop support for Ruby <=2.4 (and Rails 4?)

[TylerRick]

Support of Ruby 2.4 has ended so we should stop wasting time officially supporting it and remove it from .travis.yml.

See also rails/sprockets#672 / https://github.com/rails/sprockets/pull/604/files, which also made Ruby 2.5 the minimum version.

While we're at it, can we drop support for Rails 4, and only supporting 5.2 and up going forward?

If someone really wants to use an old version of Ruby/Rails, they can lock to an old version of xray-rails. I've already spent too much time trying to get everything (mostly tests) to work in all versions as part of #101, but I regret wasting my time doing so. The correct choice would be to just drop support for ancient versions and spend what little time we have supporting current/future versions.