Add Style/SelfAssignment Rubocop cop

The cop enforces use of self assignment operator E.g. a=a+2 gets
written as a+=2. Also the offenses listed in rubocop.todo.yml
have been corrected automatically with the --auto-correct option.
Fixes issue #1531

db/migrate/20141104131625_generate_username.rb

@@ -7,9 +7,7 @@ class GenerateUsername < ActiveRecord::Migration
     TempUser.all.each do |user|
       if user.username.blank?
         username = user.email.split('@')[0]
-        if TempUser.find_by(username: username)
-          username = username + user.id.to_s
-        end
+        username += user.id.to_s if TempUser.find_by(username: username)
         user.update_attributes(username: username)
       end
     end