Auto-correct RuboCop offenses and re-gen todo

The steps run were:

1. `rm .rubocop_todo.yml`
2. `bundle exec rubocop --auto-correct`
3. `bundle exec rubocop --auto-gen-config`

This change was made to get the Ruby code style to a more consistent
place and have RuboCop enforce offenses that were disabled due to the
.rubocop_todo.yml file.

This also updates RuboCop to ignore the vendor dir since those files
should not be style checked.

.rubocop.yml

@@ -4,6 +4,7 @@ AllCops:
   TargetRubyVersion: 2.2
   Exclude:
     - 'frontend/**/*'
+    - 'vendor/**/*'
 Style/DoubleNegation:
   Enabled: false
 Style/StringLiterals: