Adds DB constraint to avoid duplicate balloted investments

db/migrate/20170503163330_add_uniq_index_for_ballot_lines.rb

@@ -0,0 +1,5 @@
+class AddUniqIndexForBallotLines < ActiveRecord::Migration
+  def change
+    add_index :budget_ballot_lines, [:ballot_id, :investment_id], unique: true
+  end
+end