Add travis or other CI to check custom CSS on latest sphinx-rtd

[stuaxo]

Make a test to:

Install latest sphinx-rtd with the code from here on master.

Generate output for a simple function + check the markup matches what we expect so that the custom CSS will work.

[rlafuente]

do we still need this now that we build outside rtd?

[stuaxo]

It would be nice but since none of us are working on it we can close it.