Consistently use #!/usr/bin/env python

At Ksplice we used /usr/bin/python because we shipped dependencies as Debian /
Red Hat packages, which would be installed against the system Python.  We were
also very careful to use only Python 2.3 features so that even old system
Python would still work.

None of that is true at Humbug.  We expect users to install dependencies
themselves, so it's more likely that the Python in $PATH is correct.  On OS X
in particular, it's common to have five broken Python installs and there's no
expectation that /usr/bin/python is the right one.

The files which aren't marked executable are not interesting to run as scripts,
so we just remove the line there.  (In general it's common to have libraries
that can also be executed, to run test cases or whatever, but that's not the
case here.)

(imported from commit 437d4aee2c6e66601ad3334eefd50749cce2eca6)

bots/zephyr_mirror_backend.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # Copyright (C) 2012 Humbug, Inc.
 #
 # Permission is hereby granted, free of charge, to any person