Pin wheel to 0.31.1 to work around API break in 0.32.0

[hugovk]

Here's a rebuild of master, which now fails:

• https://travis-ci.org/python-pillow/pillow-wheels/builds/435068720

See:

• https://github.com/matthew-brett/multibuild/issues/202
• WheelFile moved location between 0.31.1 and 0.32.0, breaking user code pypa/wheel#255

[hugovk]

Linux builds are passing because they're installing wheel 0.31.1, but Mac builds are failing because they're installing 0.32.0.

Anyway, Matthew's working on a proper fix for multibuild/delocate now, so we may not need this workaround for Monday's release.

[agronholm]

Wheel's only public API is the command line interface.

[hugovk]

@agronholm Thanks, we don't use wheel as an API, it's used by our build dependency multibuild and its dependency delocate.

@radarhere Thanks for the update to this PR. I'll merge it so we don't need to worry for tomorrow's release.