fix python 3.3 incompatible syntax

there is a bug where source-ing the bin/activate fails with python version 3.3
(probably any 3.0+ version). This small change fixes that. Tested on python versions: 2.6, 2.7, 3.3
Merge pull request #7 from breznak/fix-py3.3

fix python 3.3 incompatible syntax (fixes #6 )