Add Python 3.7 - #90

Well, the 3.7s fail on my build:

https://travis-ci.org/hugovk/pillow-wheels/builds/397939200

I'm not sure about the macOS, but at least the Linuxes show:

The path /usr/bin/python3.7 (from --python=/usr/bin/python3.7) does not exist
/usr/bin/choose_python.sh: line 13: venv/bin/activate: No such file or directory

https://travis-ci.org/hugovk/pillow-wheels/jobs/397939217#L4931
https://travis-ci.org/hugovk/pillow-wheels/jobs/397939216#L4944

(Would probably be clearer for multibuild to fail fast at that point rather continuing and attempting to install pytest.)

(The first commit, which just updates multibuild, passes: https://travis-ci.org/hugovk/pillow-wheels/builds/397935324)

Rerunning Travis, this PR now passes

Restarted https://travis-ci.org/python-pillow/pillow-wheels/builds/397965929 and it's green now.

It had been missing 3.7 wheels for Numpy and SciPy (https://github.com/matthew-brett/multibuild/pull/182#issuecomment-401582155), now available (numpy/numpy#11418, scipy/scipy#8987).

Thanks all!