The `with sh.sudo` pattern that we were using in python-sh was
deprecated, and emperically hangs on Ubuntu xenial. Since in general
the use of python-sh/python-pbs caused trouble (requiring extra
dependencies, confusing syntax), this just removes it.
We replace it with a new zulip_tools.py library function that echoes
the command line and streams the output.
We do the same to install-phantomjs so we can remove that dependency.
This should fix a problem we've been having with errors downloading
the PhantomJS packages from their original hosting service.
Eventually we should move it to an S3 bucket.