rubocop: add rubocop-rails dependency

Otherwise we can't find many cops. I'm not sure why this is necessary
with the newer versions of the gem but not the old.

Author: peff

.rubocop.yml

@@ -1,3 +1,6 @@
+require:
+  - rubocop-rails
+
 inherit_gem:
   rubocop-github:
     - config/default.yml