Dependency housecleaning.

Summary:
Remove some unused dependencies and a bit of unused code.  A few of these, like
pyzmq, are only needed by other pip packages we use, so we don't need to
explicitly require them ourselves. Others, like htop, are useful tools but not
actually needed to use Inbox.

Test Plan:
Run a sync and `py.test tests` in a fresh VM with only the new
dependencies installed.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D218