Update test-requirements to fix sphinx build_doc

Sphinx-1.2b3 is failing with this error: "'source_dir' must be a
directory name". This change prevent that sphinx version from being
pulled, it's a backport of 6a276638

Change-Id: Ic63d776bec5c0ace7207a532c29ee193d171ee63
Closes-Bug: #1339273
Merge branch 'stable/havana' of https://github.com/openstack/horizon into stable/havana_routerrules