travis: Bump up Python version
travis: Bump up Python version

Tornado is complaining about wanting 2.7.9+. Update travis to use
the latest version (2.7.12). I couldn't find a way to make Travis
automatically use the latest minor version, I'll have another look
if this breaks again in the future.

ARM-software/trappy,ARM-software/trappy,ARM-software/trappy,ARM-software/trappy