bugfix when use osx "no such option: --with-color"

When i want to test fab with `fab test` on OSX 18.5,  display this:

```
$cd fabric
Usage: fab [options]

fab: error: no such option: --with-color
```

But on centos and ubuntu it's works. but i do not kown why, so i think when osx, i do not add '--with-color'