Install tmpreaper on jenkins to clean up the /tmp directory every day.

Summary:
Some KA tests write to /tmp and don't clean up after themselves, on
purpose (see kake/server_client.py:rebuild_if_needed().  We install
tmpreaper to clean up those files "eventually".

Note that most KA tests write to $TMPDIR, which jenkins sets to
something other than /tmp.  So we don't have to worry about cleaning
up potentially useful error files, only ones that specifically write
to /tmp (for lockfiles and the like).

Test Plan: Ran these steps manually on jenkins.

Reviewers: chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D6606