.travis.yml: Test with both Python2 and Python3

This adjusts the Travis CI test suite to test with both python2 and
python3, and also enable silent rules for the distcheck, to make
warnings more visible.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #39 from algernon/f/travis/multi-python

.travis.yml: Test with both Python2 and Python3