Revert "Run Travis on OSX."
Revert "Run Travis on OSX."

This reverts commit 90084e11579b887bdec033eb3fcc486dfdcd1f61.

ionelmc/pytest-cover,wushaobo/pytest-cov,opoplawski/pytest-cov,moreati/pytest-cov,schlamar/pytest-cov,pytest-dev/pytest-cov