Add rubocop-rails gem (Rails cops were extracted in Rubocop 0.72.0)

Rubocop was updated to v0.72.0 in 9efc6a0. That version extracted Rails
cops to a separate gem: rubocop rails.

  rubocop/rubocop#5976

This commit reintroduces the missing Rails cops.

.rubocop.yml

@@ -1,5 +1,5 @@
-Rails:
-  Enabled: true
+require:
+  - rubocop-rails
 
 AllCops:
   TargetRailsVersion: 4.0

Gemfile

@@ -39,7 +39,7 @@ end
 group :test do
   gem 'coveralls', require: false
   gem 'rails-controller-testing' # TODO: remove and replace usages
-  gem 'rubocop'
+  gem 'rubocop-rails'
   gem 'simplecov', require: false
   gem 'webmock'
 end

Gemfile.lock

@@ -213,6 +213,9 @@ GEM
       rainbow (>= 2.2.2, < 4.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 1.4.0, < 1.7)
+    rubocop-rails (2.2.1)
+      rack (>= 1.1)
+      rubocop (>= 0.72.0)
     ruby-progressbar (1.10.1)
     safe_yaml (1.0.5)
     sass (3.6.0)
@@ -288,7 +291,7 @@ DEPENDENCIES
   rails-controller-testing
   rails_12factor
   rails_admin (~> 1.4)
-  rubocop
+  rubocop-rails
   sass-rails (>= 4.0.3)
   simplecov
   skylight