BLD: wheel building failing

[jreback]

https://travis-ci.org/MacPython/pandas-wheels/jobs/435426572

cc @TomAugspurger

[jorisvandenbossche]

Issue is discussed here: https://github.com/matthew-brett/multibuild/issues/202

[bashtage]

Need to pin wheel to 0.31.1

[jreback]

build works now, only 32-bit failing: https://travis-ci.org/MacPython/pandas-wheels/builds/439958108