Now depend on mongodb 1.4.0-rc4.

The real dependency is on 1.4.0 but it isn't available yet.
1.4.0-rc4 will work until the release.
Using 1.3.x can result in duplicated events if the driver
loses connection with the mongod server or in case of a
replicaset master switch.
Merge pull request #4 from 117nord/updateMongodb

Now depends on mongodb 1.4.0-rc4.