Better fix for 'pip' detection in setup.py

We're using 'distribute', and current setup.py doesn't work with our packaging.
This fixes the problem on our end, and should be a better way of detecting whether 'pip' is being used.
This is related to this commit: https://github.com/saltstack/salt/commit/512d9adb8be4d754ab72fc0aa4f92ae69920cda1