This should speed it up a bit, as brew will download a pre-compiled "bottle", whereas pyenv compiles python from source.
The current 'python3' formula on homebrew is 3.6.0, so let's use that.
https://travis-ci.org/anthrotype/fonttools/jobs/123273641
When I do `pip install -r dev-requirements.txt` from within a virtual env,
jython sits for 3-4 minutes doing nothing, then the Travis shell exits with this cryptic error:
/usr/bin/env: python2.7: Argument list too long
