travis: improve python version test coverage

Currently travis declares ancient python 2.4 is desired. Update that to
2.6 which is the oldest version any targetted distros still needs. If we
just list a python 3 version at the top level this will double the
number of travis jobs we run which is unreasonable.

So arbitrarily pick the clang test matrix entries to build with python
3.0 and 3.6, to extend coverage of python versions, without increasing
job count or build time.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20180116134217.8725-14-berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>