Travis: Use tox for testing
Travis: Use tox for testing

It is better to have the testing command just in one place and tox is
good for that, since it is easily runnable locally too.

Refs SHUUP-2846

hrayr-artunyan/shuup,shawnadelic/shuup,suutari-ai/shoop,shawnadelic/shuup,suutari-ai/shoop,suutari/shoop,shawnadelic/shuup,suutari-ai/shoop,shoopio/shoop,suutari/shoop,shoopio/shoop,hrayr-artunyan/shuup,shoopio/shoop,hrayr-artunyan/shuup,suutari/shoop