Travis: fix MacOS py37 build

[blueyed]

This reverts commit 28dbffd.

Appears to be not necessary anymore:
https://travis-ci.org/pytest-dev/pytest/jobs/452598885#L906

[skip appveyor]

[nicoddemus]

Note: [skip appveyor] needs to be on the "subject" line of the commit:

Add [skip ci] or [ci skip] to commit message title and build won’t be triggered by AppVeyor for that commit. Or explicitly skip AppVeyor only with [skip appveyor] and still allow any other CI to build the commit (eg. Travis CI).
NOTE: Similarly to custom skip commit message, AppVeyor searches for [skip ci] / [ci skip] / [skip appveyor] in the commit message title only. This avoids confusion when squashing commits, in particular with the GitHub squash-and-merge feature.

[nicoddemus]

Cancelling AppVeyor manually to free the queue.