Roll back python version dependency that does not support site packages.
Roll back python version dependency that does not support site packages.

https://docs.travis-ci.com/user/languages/python/#Travis-CI-Uses-Isolated-virtualenvs

woutervanwijk/Mopidy-MusicBox-Webclient,woutervanwijk/Mopidy-MusicBox-Webclient,pimusicbox/mopidy-musicbox-webclient,woutervanwijk/Mopidy-MusicBox-Webclient,pimusicbox/mopidy-musicbox-webclient,pimusicbox/mopidy-musicbox-webclient