travis: Add 2.7 to build matrix. Use stable ruby for rails-master
travis: Add 2.7 to build matrix. Use stable ruby for rails-master

Adds Ruby 2.7 so tests are run against it. It appears Rails 5.1+ will
run on Ruby 2.7. I've also switched rails-master to use whatever the
current stable version of ruby is which should avoid having to update
the line again in the future.

integrallis/stripe_event,fullfabric/stripe_event,fullfabric/stripe_event,integrallis/stripe_event