Bug 885233: Use python2 in activation script on *nix

This is another attempt at [Bug 885233](https://bugzilla.mozilla.org/show_bug.cgi?id=885233). Unlike PR #1088, I'm just making use of PEP394's requirement that `python2` exist and be a Python 2.x interpreter. No need to even bother with the `python` program if the sdk is version-dependant.