Remove dependency on activemodel

This dependency has been introduced by John Firebaugh in commit #4549965af169019c7f3d679371e4650057d176b2 because `lib/rspec/rails/mocks.rb` required it.

But this file was removed in commit #29c175832f55d180473f35c22d1b88245a828917.

Currently rspec-rails will check for defined?(::ActiveModel) before patching/extending it.

The dependency on activemodel will install unnecessary gems for some applications and will also trigger some bugs in Bundler which would prevent pre releases of railties to be installed with rspec-rails.