Set Travis to use Python 2.7
Created by: Ratfink
The default language is Ruby, which causes Travis to waste about four seconds setting up a Ruby environment we don't need every time CI is run. This PR changes the language to the one we actually use, Python 2.7.
Like https://github.com/KiCad/kicad-symbols/pull/370, but with a cleaner commit history because I already figured out what needs to be done.