Revert ".travis: go back to using the "os" block"

This reverts commit eb23fa86deb7d7043f37113f2e4f7ea50a02c443.

While "osx" mode now works with the NodObjC repo, `nvm` is not
installed on the OS X test boxes at this time. I'm working with
the Travis-CI support team to get that fixed.