Change shebang on mongodb init script to bash

The initscript uses [[, which is only available on bash, not sh.

This should fix travis-ci/travis-ci#788.

ci_environment/mongodb/files/default/mongodb.sysvinit.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # init.d script with LSB support.
 #