This is important for both ensuring the Nagios checks work correctly
in production, as well as making sure the `zulip` user can access the
virtualenv (owned by the `travis` user) in Travis CI.
Apparently, c74a74dc74 introduced a bug
where we are no longer correctly depending on build-essential as part
of the Zulip development environment installation process.
Fixes#1111.
This is needed because hash_reqs.py is used to create a virtualenv.
Currently we only use virtualenv in development, but we will soon
start using it in production. Scripts used in production should be
put in scripts/.
Eklavya Sharma
Tim Abbott