gems: add Rubocop gem.

Gemfile.lock

@@ -74,6 +74,7 @@ GEM
       public_suffix (>= 2.0.2, < 5.0)
     airbrussh (1.4.0)
       sshkit (>= 1.6.1, != 1.7.0)
+    ast (2.4.0)
     aws-eventstream (1.1.0)
     aws-partitions (1.308.0)
     aws-sdk-core (3.94.0)
@@ -171,6 +172,7 @@ GEM
     i18n (1.8.2)
       concurrent-ruby (~> 1.0)
     ipaddress (0.8.3)
+    jaro_winkler (1.5.4)
     jmespath (1.4.0)
     jquery-rails (4.3.5)
       rails-dom-testing (>= 1, < 3)
@@ -245,6 +247,9 @@ GEM
       multi_json (~> 1.3)
       multi_xml (~> 0.5)
       rack (>= 1.2, < 3)
+    parallel (1.19.1)
+    parser (2.7.1.2)
+      ast (~> 2.4.0)
     pg (1.2.3)
     pg (1.2.3-x64-mingw32)
     pry (0.13.1)
@@ -297,6 +302,7 @@ GEM
       method_source
       rake (>= 0.8.7)
       thor (>= 0.20.3, < 2.0)
+    rainbow (3.0.0)
     raindrops (0.19.1)
     rake (13.0.1)
     rakismet (1.5.4)
@@ -313,6 +319,20 @@ GEM
       actionpack (>= 5.0)
       railties (>= 5.0)
     retriable (3.1.2)
+    rexml (3.2.4)
+    rubocop (0.82.0)
+      jaro_winkler (~> 1.5.1)
+      parallel (~> 1.10)
+      parser (>= 2.7.0.1)
+      rainbow (>= 2.2.2, < 4.0)
+      rexml
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (>= 1.4.0, < 2.0)
+    rubocop-rails (2.5.2)
+      activesupport
+      rack (>= 1.1)
+      rubocop (>= 0.72.0)
+    ruby-progressbar (1.10.1)
     ruby-vips (2.0.17)
       ffi (~> 1.9)
     ruby2_keywords (0.0.2)
@@ -367,6 +387,7 @@ GEM
       unf_ext
     unf_ext (0.0.7.7)
     unf_ext (0.0.7.7-x64-mingw32)
+    unicode-display_width (1.7.0)
     unicorn (5.5.5)
       kgio (~> 2.6)
       raindrops (~> 0.7)
@@ -449,6 +470,8 @@ DEPENDENCIES
   request_store
   responders
   retriable
+  rubocop
+  rubocop-rails
   ruby-vips
   rubyzip
   sanitize