RF: switch travis tests to use virtualenvs

We were using a trick to get the dependencies on travis, working round their
virtualenv setup.

Now there is a usable travis wheel repository, we can use our own virtualenvs
and install all optional dependencies.

Also allows checking minimal dependencies, and Pythons 2.6 through 3.4.
FYI It might help to increase the timeout if the network is not very reliable:

```
pip install --timeout=60 ...
```

otherwise some noticed unstable build with the default timeout of 15 seconds.