Prefer Buck's twitter-commons over a system-installed one.

Summary:
We were properly prepending the path for setuptools, but not for
twitter-commons, so the system version would be used instead of ours.

Fixes #175

Test Plan: `buck test --all`
Fix typo flagged by @davido.

Summary: Addresses https://github.com/facebook/buck/commit/24f16c8e012000a86ccba564fb0be84504b60824#commitcomment-7327662.

Test Plan: Sandcastle.