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.
RF: refactor coverage in travis file

Coverage went wonky with virtualenvs
TST: add verbose flag to travis test output

Use standard nosetests, bug for setuptools we were working round was
fixed a while ago in nose.
Merge pull request #474 from matthew-brett/refactor-travising-2

RF: switch travis tests to use virtualenvs