Add RuboCop

* Exclude bin/ because it's generated code
* Max line length of 120 characters
* Documentation not required
* Prefer double-quoted strings

.rubocop.yml

@@ -0,0 +1,15 @@
+Rails:
+  Enabled: true
+
+AllCops:
+  Exclude:
+    - bin/*
+
+Metrics/LineLength:
+  Max: 120
+
+Style/Documentation:
+  Enabled: false
+
+Style/StringLiterals:
+  EnforcedStyle: double_quotes

Gemfile

@@ -61,6 +61,7 @@ end
 
 group :development, :test do
   gem 'pry'
+  gem "rubocop", require: false
 end
 
 group :test do

Gemfile.lock

@@ -57,6 +57,7 @@ GEM
       childprocess (>= 0.3.6)
       cucumber (>= 1.1.1)
       rspec-expectations (>= 2.7.0)
+    ast (2.3.0)
     autoprefixer-rails (5.1.0)
       execjs
       json
@@ -158,6 +159,9 @@ GEM
       cocaine (~> 0.5.5)
       mime-types
       mimemagic (= 0.3.0)
+    parser (2.3.1.2)
+      ast (~> 2.2)
+    powerpack (0.1.1)
     pry (0.10.1)
       coderay (~> 1.1.0)
       method_source (~> 0.8.1)
@@ -191,12 +195,20 @@ GEM
       activesupport (= 4.2.6)
       rake (>= 0.8.7)
       thor (>= 0.18.1, < 2.0)
+    rainbow (2.1.0)
     rake (11.1.2)
     ref (1.0.5)
     rspec-expectations (3.1.2)
       diff-lcs (>= 1.2.0, < 2.0)
       rspec-support (~> 3.1.0)
     rspec-support (3.1.2)
+    rubocop (0.42.0)
+      parser (>= 2.3.1.1, < 3.0)
+      powerpack (~> 0.1)
+      rainbow (>= 1.99.1, < 3.0)
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (~> 1.0, >= 1.0.1)
+    ruby-progressbar (1.8.1)
     rubyzip (1.1.7)
     safe_yaml (1.0.4)
     sanitize (3.0.3)
@@ -253,6 +265,7 @@ GEM
     uglifier (2.7.2)
       execjs (>= 0.3.0)
       json (>= 1.8.0)
+    unicode-display_width (1.1.0)
     uniform_notifier (1.6.2)
     websocket (1.2.2)
     will_paginate (3.0.7)
@@ -293,6 +306,7 @@ DEPENDENCIES
   rails-dom-testing!
   rails_autolink
   rspec-expectations
+  rubocop
   sanitize (>= 3.0.0)
   sass-rails (~> 5.0)
   selenium-webdriver (>= 2.50)