Fixed issue #416

The commits are pretty self-explaining. I fixed the documentation testing.
The next step is to fix issue #414 to make Travis CI fail if docs have warnings.

I didn't get one thing: what are the 'doc-requirements.txt' for? It seems there are too many things there.
To 'make docs' the only dependencies are:
Jinja2==2.7.3
MarkupSafe==0.23
Pygments==1.6
Sphinx==1.2.2
docutils==0.11

And to test them, at the moment the only additional dependency is 'certifi'.