The zulip and zulip_botserver packages specify mock as a runtime
dependency, which is only needed for testing during development.
So, it made more sense to move it to requirements.txt so that
it can be installed by ./tools/provision during development.
In zulip_bots/setup.py, we now don't specify a minimum version when
checking for dependency on the zulip package. We just want the
latest one.
In zulip_botserver/setup.py, we now don't specify a minimum version
when checking for dependencies on the zulip and zulip_bots package.
We just want the latest ones.