Add `DOWNTIME = false` to db/migrate/20160615142710_add_index_on_requested_at_to_members

Signed-off-by: Rémy Coutable <remy@rymai.me>

db/migrate/20160615142710_add_index_on_requested_at_to_members.rb

 class AddIndexOnRequestedAtToMembers < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
+  DOWNTIME = false
+
   disable_ddl_transaction!
   def change