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.

mach

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2.7
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.