Try to fix Travis

* Allow installing of requirements.txt to not fail the build by piping it to `true`.
* Add pypy3 which is also tested with Limnoria.
* Comment alllow_failures as the only failure was in requirements.txt.

If you are wondering why I used `pip install -vr requirements.txt||true` instead of `pip install -vr requirements.txt || true`, it's because Travis transformed spaces to some weird characters and failed the build with command not found for strange characters.