Fixed multimap to compile with GCC 4.7.

From: Brenden Matthews <brenden@diddyinc.com>
Review: https://reviews.apache.org/r/10561

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1470040 13f79535-47bb-0310-9956-ffa450edef68