Update to a more recent version of `six.py`.

From here: https://bitbucket.org/gutworth/six/overview
Merge pull request #213 from arokem/fix_six

RF - Update to a more recent version of `six.py`.
ENH: Non-linear least squares tensor fitting.
ENH: First-pass implementation of RESTORE.
RF + TST: RESTORE.
RF: Simplified the tensor decomposition.
DOC: Small comment change.
TST: Make the tests more runnable.
RF: Silence some runtime warnings.
RF: Replace fmin with leastsq.
TST: update the test to the new API.
RF: Get rid of this cruft.
DOC: Describe this data set.
BF: Reshape the params before returning them.
WIP: Beginning implementation of the Jacobian.

THIS DOES NOT WORK YET. STILL NEED TO THINK ABOUT IT.
RF: Move un-necessary operation down.

This should give us some speed-up :-)
RF: Reimplement the Jacobian based on equations in the Koay paper.
RF - streamline Jacobian calculation and add test
TST: use the right function for the job.
BF: python3 compatibility.