This has been fixed upstream (http://bugs.python.org/issue26083) but who
knows when Apple will update their version of Python they distribute so
this change simply works around the issue by treating a ValueError to be
the same as an OSError. This fix seems smaller/safer then switching to 
the subprocess32 package as it adds another external dependency an the 
documentation states that it hasn't been tested on Windows.

Before my fix, when running rbt post, I get:
  ValueError: insecure string pickle
(I don't have tfs installed)

After my fix, running rbt post works fine.