Removed unnecessary "config.mock_with :rspec" from spec_helper.rb

This statement configures RSpec to do what it would do anyway, and
triggers errors regarding RSpec.configure being called after examples
have been defined.

See: rspec#371
See: rspec/rspec-core#455

lib/generators/rspec/install/templates/spec/spec_helper.rb

@@ -16,7 +16,6 @@
   # config.mock_with :mocha
   # config.mock_with :flexmock
   # config.mock_with :rr
-  config.mock_with :rspec
 
   # Remove this line if you're not using ActiveRecord or ActiveRecord fixtures
   config.fixture_path = "#{::Rails.root}/spec/fixtures"