Make reference to `ActiveModel` module absolute

When rspec-activemodel-mocks is around RSpec::ActiveModel will
exist so that will be resolved instead of the actual ActiveModel
module.
Squelch model mocking deprecation messages

Only include `stub_model` and `mock_model` from rspec-rails if
rspec-activemodel-mocks is not providing them. We need to check if this
is already happening in rspec-rails because rspec-activemodel-mocks does
not depend on rspec-rails. This leads to the rails default of running
`Bundler.require` including rspec-activemodel-mocks before rspec-rails
so rspec-activemodel-mocks is unable to overwrite the methods provided
by rspec-rails