Removed hard coded path of python app on mac platform

I am trying to make the **./bootstrap_develop.sh** work on mac, one of the problems I've had is that the bitmask_client **postmkvenv.sh** is trying to use a hardcoded python (with the path used by macports) to get the python lib.

Most people I know use homebrew instead and that would be yet another path, so I think the best way to go is using the python directly (considering mac comes with python installed since forever).