CMake actually marks that we require six 1.4.0, however, I can't find
any packages anywhere for 1.4.0, and the lowest version I've seen
requested is 1.5.2.

This fixes requirements for working with six 1.5.2, and sets tox to use
1.5.2 (and a suitable version of mock). Primarily there are a few things
we're using that are not available: six.moves.getcwd, six.viewvalues,
six.python_2_unicode_compatible.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Tested-by: Brian Paul <brianp@vmware.com>