Pin version of sphinx<1.6.0
Pin version of sphinx<1.6.0

Sphinx 1.6.0 and 1.6.1 have landed today and there is a commit that
breaks pbr [1].

Let's pin for now.

[1]: https://github.com/sphinx-doc/sphinx/commit/2afa0b6627f7e5afb188d5a60c8c4767f6250774

Change-Id: Ic0cf50db0e36489ae137882f60d8e2ede9212725

rdo-infra/weirdo,redhat-openstack/weirdo