Add 'mock' requirement for zulip and zulip_botserver.

These packages depend on mock. Previously, that dependency
was satisfied "by chance" because zulip_bots happened to
install 'mock'.

zulip/setup.py

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

zulip_botserver/setup.py

@@ -38,6 +38,7 @@ setuptools_info = dict(
         'zulip',
         'zulip_bots',
         'flask>=0.12.2',
+        'mock',
     ],
 )