Fix Travis CI for MacOSX and Python 3 support

[pombredanne]

Following up on @sschuberth PR #5 there is something fishy in the travis setup: the tests run on Mac and Linux but only on Python 2.7 and Python 3.4 and 3.5 are ignored.

[pombredanne]

This has been fixed with f4c4270#diff-354f30a63fb0907d4ad57269548329e3 and 0d1af80#diff-354f30a63fb0907d4ad57269548329e3

It should have been a separate commit though ...