Corrections to requirements.txt

1) Corrected format, should be == instead of =
2) The specified Zinnia version wasn't in PyPI, so updated to the project's git repo, head as of this commit
3) The specified Grappelli version wasn't in PyPI, so updated to the project's git repo, stable 2.4.x branch
4) Ran through pip freeze, which added a few references.  This sets up the file for easy freezing later when you add packages