Bug 957721 - Switch mach's shebang to look for python2.7; r=glandium

OpenBSD doesn't have "python" on $PATH by default. "python" may also
come from a non-2.7 Python. Switching the shebang will help ensure the
user invokes mach with Python 2.7.

There is a risk this may break mach for some people. If the masses
complain, we may roll back.

--HG--
extra : rebase_source : e05b707779bc80e9f9c9f30ba8cd70aa6aa9847f
extra : amend_source : 418ecc9e4c289e7dd8c4423e701ebc9574ecae45