Update mongodb to 2.6.4.

Changes:
- Fix for text index where under specific circumstances, in-place updates
  to a text-indexed field may result in incorrect/incomplete results
- Check the size of the split point before performing a manual split chunk
  operation
- Ensure read preferences are re-evaluated by drawing secondary connections
  from a global pool and releasing back to the pool at the end
  of a query/command
- Allow read from secondaries when both audit and authorization are enabled
  in a sharded cluster

All issues fixed:

https://jira.mongodb.org/issues/?jql=project%20%3D%20SERVER%20AND%20fixVersion%20%3D%20%222.6.4%22