Make readthedoc expose the right version of the documentation

[Martiusweb]

Documentation is pushed on readthedocs, but the latest commit on master is always the one used for the documentation. It means that the documentation may describe unreleased features, and it's not possible to browse older versions.

The default version should be the latest stable (the newest tag starting with a "v"), and each version should be activated. Maybe this feature exists somewhere in readthedoc, or maybe this can be scripted with a readthedoc api called with a post-deploy script in travis.