Add Python3 to TravisCI testing

[tcstewar]

To make sure we keep Python3 support, we should also run our tests with it (like nengo does).

[krisfrenette]

@tbekolay is this something that is still needing to be done

[tbekolay]

The TravisCI testing isn't working for Python 2 or 3, so this is still an issue, though not sure it'll end up getting fixed.