Require sphinx < 2, sphinxcontrib_images does not support sphinx 2.

requirements.txt

@@ -2,7 +2,7 @@
 # should match https://pypi.python.org/pypi names. For the full spec or
 # dependency lines, see https://pip.readthedocs.org/en/1.1/requirements.html
 vcversioner
-sphinx >= 1.3
+sphinx >= 1.3, < 2
 sphinxcontrib-httpdomain
 sphinxcontrib-images
 sphinxcontrib-programoutput