Update binary_util OS map for OSX Sierra.

This is in anticipation of posting the rebuilt
binaries for 10.12. This will land in master as
a followup.

Testing Done:
Ran the unit tests locally - CI is green.

Hope to get the binaries up tonight, I finished installing Sierra this morning.
My home MacBook was, um, two major versions behind.

https://travis-ci.org/pantsbuild/pants/builds/163136142

Bugs closed: 3910

Reviewed at https://rbcommons.com/s/twitter/r/4266/

src/python/pants/binaries/binary_util.py

@@ -33,6 +33,7 @@
   ('darwin', '13'): ('mac', '10.9'),
   ('darwin', '14'): ('mac', '10.10'),
   ('darwin', '15'): ('mac', '10.11'),
+  ('darwin', '16'): ('mac', '10.12'),
 }