Bug 792202 - Make mozboot work with Python 2.6.1; r=jhammel DONTBUILD

Python 2.6.1 is what ships on OS X 10.6 and we want to support mozboot
there.

We include 3 fixes:

1) optparse doesn't like unicode as its initial argument
2) __init__(**kwargs) doesn't like unicode keys
3) subprocess does not have check_output

We took the easy solution of removing unicode_literals. This should have
no significant consequences.

python/mozboot/bin/bootstrap.py

@@ -10,7 +10,9 @@
 # bootstrap support. It does this through various means, including fetching
 # content from the upstream source repository.
 
-from __future__ import print_function, unicode_literals
+# If we add unicode_literals, optparse breaks on Python 2.6.1 (which is needed
+# to support OS X 10.6).
+from __future__ import print_function
 
 import os
 import shutil