[travis] test on python 3.5 instead of 3.6 on osx
[travis] test on python 3.5 instead of 3.6 on osx

That's the one which is installed with `brew install python3` when
we don't update Travis' homebrew.

3.5 is good enough, we already test 3.6 on Linux anyway, and most
developers run test locally on their macs with python3.6 anyway

googlefonts/fonttools,fonttools/fonttools