Update supported RSpec versions.

Rails 4.2.0 has been officially released. Drop usage of the RC line and
use the main releases.

Rails `master` is now targeting Rails 5.x. Plans for Rails 5.x support
with RSpec is not official yet. Until we determine our planned support
line drop support for Rails `master` on rspec-rails `master`.
Start supporting Ruby 2.2
Rails 4.2.0 must pass now.
Merge pull request #1259 from rspec/build-matrix-update-rails-4_2

Update supported RSpec versions.