change mongo version to 2.6.4

@jzoldak  @feanil  Please review.  I've tested this on devstack.  The mongo role is working, but I'm getting some weird, seemingly unrelated errors afterward.

Once this is merged,  I can update the jenkins worker AMI.
For reasoning behind this: http://docs.mongodb.org/manual/tutorial/install-mongodb-on-ubuntu/#considerations

"""Changed in version 2.6: The package structure and names have changed as of version 2.6. For instructions on installation of an older release, please refer to the documentation for the appropriate version."""