tar.gz targets look in DEPS_DIR instead of deps

Summary:
When projects set a different DEPS_DIR than leptus,
the tar.gz will be curl-ed to that instead.
This change prevents pulling down the tar.gz every time.

If this looks good, I'll change newton_sevice to use my fork,
and submit a PR to leptus to see what he thinks.

Test Plan:
Changed newton_service Makefile to use my fork and this SHA

Wiped newton/deps, build once, and rebuilt to ensure nothing new was pulled down

Reviewers: srmerritt, dana.powers

Differential Revision: https://phabricator.rdio.com/D15060