Move to github actions

[hahmed]

What

Move to GitHub actions and drop TravisCI.

• Gemfile has been update to point to rails:main because rails:master no longer exists
• I have locked rack to a specific commit because the latest Rack version breaks on rails see Remove unnecessary call to super() in Rack::Request::Env rack/rack#1751
• The method signature has been updated for ActionView::Base.new, we now pass in {}, null for the locals and controller respectively.
• The attribute media="screen" which has now been removed see Remove legacy default media=screen from stylesheet_link_tag rails#41215 is needed for the tests to pass for Rails versions < 7. Rather than instantiation a rails app and setting the config Rails.application.config.action_view.stylesheet_media_default=true, this seems like the easier fix.

Anything else

I have split into 2 commits, 1 for the build (which is currently failing on travis ci) and the other for the test fixes, but let me know if you want me to squash into 1 commit.

The GitHub actions are not being run here, but you can see them all green here https://github.com/hahmed/sprockets-rails/actions/runs/952618642 -- not sure why I can see the actions workflow on my fork but not upstream, but @byroot mentioned if we merged this PR we would prob see the actions here too.

Should we consider updating the target ruby version in a follow up PR, perhaps ruby 2.7?

[hahmed]

fyi... rack/rack#1755 reverts Flatten Rack::Request::Env into Request we could target the maser branch for Rack once that get's merged.