requirements: Remove runtime dependency on mock.

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.

zulip/setup.py

@@ -55,7 +55,6 @@ setuptools_info = dict(
     install_requires=['requests>=0.12.1',
                       'six',
                       'typing>=3.5.2.2',
-                      'mock',
                       ],
 )