Update readthedocs python version to 3.5

[Tjstretchalot]

I'm pretty sure the documentation build is failing because it's using python 2.7. It has an option to use python 3.5 which worked on my fork (its under Admin).

Link to builds: http://readthedocs.org/projects/pygorithm/builds/

[Tjstretchalot]

Hmm builds started succeeding again.