Remove migrations warning

DEPRECATION WARNING: Directly inheriting from ActiveRecord::Migration
is deprecated. Please specify the Rails release the migration was
written for:

class MigrationClass < ActiveRecord::Migration[4.2]

(called from require at bin/rails:4)

db/migrate/20170822144743_add_community_to_budget_investments.rb

@@ -1,4 +1,4 @@
-class AddCommunityToBudgetInvestments < ActiveRecord::Migration
+class AddCommunityToBudgetInvestments < ActiveRecord::Migration[4.2]
   def change
     add_reference :budget_investments, :community, index: true, foreign_key: true
   end