Update "Developers" Section in acknowledgements.rst

[connorjbracy]

The source file acknowledgements.rst has a section "Developers" which is not an accurate list of active developers.

@sbillinge, when you get a chance, take a look at the list of members listed as "The active DiffPy development team" and let me know who should be moved to the "Former members of the project" section.

Also, I assume I should add both myself and Zach as active developers, correct?