Use "/usr/bin/env python2" for all shebangs

This fixes https://github.com/kliment/Printrun/issues/325

If ever this patch no longer cleanly applies, or you wish to apply it to experimental, it can be reproduced with the command:
 
    grep -rl '^#!' * | xargs sed -i 's|#!.*|#!/usr/bin/env python2|'