Make multithreading work on linux

Summary:
Before I had to manually use taskset to keep
all of the multiprocessing threads from running on one
core. Now it just works (tm)
This is the one to review, gentlemen. My bad.

Test Plan: train new models

Reviewers: jace, jascha

Reviewed By: jace

Differential Revision: http://phabricator.khanacademy.org/D4124