make the migration reversible

changelogs/unreleased/remove-ensure-ref-fetched-from-controllers.yml

 ---
 title: Removed merge requests fetching their refs on every action.
-merge_request:
+merge_request: 15129
 author:
 type: removed

db/post_migrate/20171101134435_remove_ref_fetched_from_merge_requests.rb

@@ -9,6 +9,6 @@ class RemoveRefFetchedFromMergeRequests < ActiveRecord::Migration
   #
   # See https://gitlab.com/gitlab-org/gitlab-ce/issues/36061
   def change
-    remove_column :merge_requests, :ref_fetched
+    remove_column :merge_requests, :ref_fetched, :boolean
   end
 end