Add and apply Performance/Sum rubocop rule

We're not adding it for performance reasons but because it simplifies
the code.

lib/remote_translations/microsoft/sentences_parser.rb

@@ -18,6 +18,6 @@ module RemoteTranslations::Microsoft::SentencesParser
   end
 
   def characters_count(texts)
-    texts.map(&:size).reduce(:+)
+    texts.sum(&:size)
   end
 end

lib/tasks/proposal_actions.rake

@@ -384,7 +384,7 @@ namespace :proposal_actions do
       5
     ]
 
-    votes_count = expected_supports.reduce(0.0) { |sum, x| sum + x }
+    votes_count = expected_supports.sum
     goal_votes = Setting["votes_for_proposal_success"].to_f
     cached_votes_up = 0