PDF build support >= Sphinx 1.6.1
PDF build support >= Sphinx 1.6.1

On Dependencies in Sphinx "Release 1.6.1"
(link: http://www.sphinx-doc.org/en/stable/changes.html ),
latexmk is needed to build PDF files.

This patch specifies latexmk package in bindep.txt
not to fail PDF builds.

Change-Id: I858b63f00c12b66247346c42451d3fd37df19702

openstack/openstackdocstheme,openstack/openstackdocstheme,openstack/openstackdocstheme,openstack/openstackdocstheme