Remove deleted milestones from merge requests

Closes https://github.com/gitlabhq/gitlabhq/issues/9643

db/migrate/20150918161719_remove_invalid_milestones_from_merge_requests.rb

+class RemoveInvalidMilestonesFromMergeRequests < ActiveRecord::Migration
+  def up
+    execute("UPDATE merge_requests SET milestone_id = NULL where milestone_id NOT IN (SELECT id FROM milestones)")
+  end
+end