RF: make .travis.yml use travis Python and wheels

We were using erlang as language and apt-get for dependencies, because of the
difficulty getting numpy, scipy onto the travis slaves when using the standard
travis Python virtualenvs.

Now travis has numpy in the virtualenvs, and astropy, nipy.bic have wheels for
the travis slaves, so installation is much quicker.  This commit switches back
to using the travis Python testing machinery.