Travis CI: make specified Python versions usable on jobs

For the two Python jobs, which seem to have the goal of making sure
QEMU builds successfully on the 3.0-3.6 spectrum of Python 3 versions,
the specified version is only applicable if a Python virtual
environment is used.  To do that, it's necessary to define the
(primary?) language of the job to be Python.

Also, Travis doesn't have a 3.0 Python installation available for the
chosen distro, 3.4 being the lower version available.

Reference: https://docs.travis-ci.com/user/languages/python/#specifying-python-versions


Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20181109150710.31085-4-crosa@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
[ehabkost: Now 3.4 is the lowest Python version available]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

fixup! Travis CI: make specified Python versions usable on jobs

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>