Install rubocop in development mode.

Gemfile

@@ -151,6 +151,7 @@ group :development do
   gem 'binding_of_caller'
   gem 'annotate'
   gem 'foreman', require: false
+  gem 'rubocop', require: false
 end
 
 # this is an optional gem, it provides a high performance replacement

Gemfile.lock

@@ -225,7 +225,11 @@ GEM
     openid-redis-store (0.0.2)
       redis
       ruby-openid
+    parallel (1.11.2)
+    parser (2.4.0.0)
+      ast (~> 2.2)
     pg (0.20.0)
+    powerpack (0.1.1)
     progress (3.3.1)
     pry (0.10.4)
       coderay (~> 1.1.0)
@@ -274,6 +278,8 @@ GEM
       activesupport (= 4.2.9)
       rake (>= 0.8.7)
       thor (>= 0.18.1, < 2.0)
+    rainbow (2.2.2)
+      rake
     raindrops (0.18.0)
     rake (12.0.0)
     rake-compiler (1.0.4)
@@ -314,10 +320,18 @@ GEM
       rspec-support (~> 3.6.0)
     rspec-support (3.6.0)
     rtlit (0.0.5)
+    rubocop (0.49.1)
+      parallel (~> 1.10)
+      parser (>= 2.3.3.1, < 3.0)
+      powerpack (~> 0.1)
+      rainbow (>= 1.99.1, < 3.0)
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (~> 1.0, >= 1.0.1)
     ruby-ll (2.1.2)
       ansi
       ast
     ruby-openid (2.7.0)
+    ruby-progressbar (1.8.1)
     ruby-readability (0.7.0)
       guess_html_encoding (>= 0.0.4)
       nokogiri (>= 1.6.0)
@@ -373,6 +387,7 @@ GEM
     unf (0.1.4)
       unf_ext
     unf_ext (0.0.7.4)
+    unicode-display_width (1.3.0)
     unicorn (5.3.0)
       kgio (~> 2.6)
       raindrops (~> 0.7)
@@ -465,6 +480,7 @@ DEPENDENCIES
   rspec-html-matchers
   rspec-rails
   rtlit
+  rubocop
   ruby-readability
   sanitize
   sassc