Remove python2 from travis-ci
Remove python2 from travis-ci

python2 is no longer supported by girder, so the travis-ci tests are
failing.

Remove python2 from testing.

We should eventually remove python2 support entirely, but that is being
tracked [here](https://github.com/Kitware/HPCCloud/issues/637).

Signed-off-by: Patrick Avery <743342299f279e7a8c3ff5eb40671fce3e95f13a@kitware.com>

Kitware/HPCCloud,Kitware/HPCCloud,Kitware/HPCCloud,Kitware/HPCCloud