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

app/models/event.rb

@@ -115,7 +115,7 @@ class Event < ActiveRecord::Base
   def average_rating
     @total_rating = 0
     votes.each do |vote|
-      @total_rating = @total_rating + vote.rating
+      @total_rating += vote.rating
     end
     @total = votes.size
     @total_rating > 0 ? number_with_precision(@total_rating / @total.to_f, precision: 2, strip_insignificant_zeros: true) : 0