Various fixes.

Summary:
+ Fix build on Linux. Linux builds require linking with -lrt.
+ Add Vagrantfile for testing on Ubuntu. This most accurately reflects the environment that Travis-CI uses (and caught the linker bug above).
+ Add func to remove the DB. Use within tests.

Test Plan: unit tests. linux/darwin.

Reviewers: spencerkimball

Reviewed By: spencerkimball

Subscribers: team

Differential Revision: http://phabricator.andybons.com/D48