DEV: Stop polluting all Ruby classes

The `ReviewableScore` model was defining class methods on `self.class`
from a singleton context so instead of defining methods on
`ReviewableScore` it was defining them on `Class`, so basically on every
existing class.

This patch resolves this issue. Using `enum` from `ActiveRecord` in the
future will avoid this kind of problems.

app/models/reviewable_score.rb

@@ -49,7 +49,7 @@ class ReviewableScore < ActiveRecord::Base
   # Generate `pending?`, `rejected?`, etc helper methods
   statuses.each do |name, id|
     define_method("#{name}?") { status == id }
-    self.class.define_method(name) { where(status: id) }
+    singleton_class.define_method(name) { where(status: id) }
   end
 
   def score_type