move pymongo extension to 2.3

Summary: Since we have upgraded mongo db to latest stable recently it would be nice to be able to use its new features. One of them is Aggregation Framework which requires pymongo 2.3. We also use pymongo 2.3 in main webapp hence I hope this change should not break stuff. However, in rare occasion that this breaks things I will be responsible for fixing them.

Test Plan: external package change

Reviewers: chris, jace

Reviewed By: chris

CC: james

Differential Revision: http://phabricator.khanacademy.org/D2722