Updating mongodb client to 1.3.18, 1.3.17 has memory leak

v1.3.17 has a memory leak, it was fixed 2 days ago.

the commit is here: https://github.com/mongodb/node-mongodb-native/commit/8a31908c6ae0e0de5c93bb3a39a847ca89c6cdc3
Merge pull request #13 from wescleveland/patch-1

Updating mongodb client to 1.3.18, 1.3.17 has memory leak