fix syntax error in pythonbrew/etc/bashrc

After upgrading to Ubuntu 14.04, dash is not happy, see [this stackoverflow question](http://stackoverflow.com/questions/22768854/pythonbrew-bashrc-awk-line-no-longer-works). [This answer](http://stackoverflow.com/a/22774174/1582182) from user [Ed Morton](http://stackoverflow.com/users/1745001/ed-morton) solves the issue.