Update mongodb to 2.4.9.

Notable changes:

2.4.9 - January 10, 2014
- Fix for instances where mongos incorrectly reports a successful
  write SERVER-12146.
- Make non-primary read preferences consistent with slaveOK versioning
  logic SERVER-11971.
- Allow new sharded cluster connections to read from secondaries when
  primary is down SERVER-7246.

2.4.8 - November 1, 2013
- Increase future compatibility for 2.6 authorization features SERVER-11478.
- Fix dbhash cache issue for config servers SERVER-11421.

2.4.7 - October 21, 2013
- Fixed over-aggressive caching of V8 Isolates SERVER-10596.
- Removed extraneous initial count during mapReduce SERVER-9907.
- Cache results of dbhash command SERVER-11021.
- Fixed memory leak in aggregation SERVER-10554.