Add ruby 2.7.1 and update patch releases for existing rubies.

Rails 5.2 has an unreleased fix for the use of the &. operator.

This gem supports a lot of Ruby/Rails versions so we'd need to bump
the major version if we wanted to break backwards compatibility.

Compressors should respond to `call` otherwise Sprockets will print
a deprecation warning about legacy processing interfaces.

Rails 6.0+ prints a deprecation warning when calling
ActionView::Base.new without a lookup context.

We know they're going away but they still need testing for now.

In rails/rails@1165401 the Active Support core extension for
Kernel#class_eval was removed so we need to call it on the
singleton_klass ourselves.