Set a default timezone if the environment doesn't provide one

Summary: See comment.

Test Plan: Deleted my env timezone. Ran a script with strtotime(), got yelled at. Applied patch, strtotime() works cleanly.

Reviewers: Makinde, btrahan

Reviewed By: Makinde

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2000