Summary:
Remove the third-party/ directory and no longer rely on copies of dependencies
in the repo. When needed, fetch dependencies normally. This can usually be
accomplished via `pip install -r requirements.txt` or `pip install -r
requirements-dev.txt`.
Even though `peru.yaml` is gone, `.peru/` is still in `.gitignore`, because the
tests check for untracked files and will detect `.peru/` as an error.
Test Plan: Successfully ran tests locally and via Travis CI.
Reviewers: jacko
Reviewed By: jacko
Differential Revision: https://phabricator.buildinspace.com/D145
Jack O'Connor