Require django-pipeline 1.2.24.

Right now, Djblets only works with django-pipeline 1.2.24. This sets the
requirement to be exactly that version, to prevent issues with pulling
in 1.3.x.
Revert "Change django.utils.simplejson to json."

This reverts commit 992fedd0b3036566f9084a1e1c5e7d793e284be2.

This change is being moved to the master branch, as Djblets 0.7.x still
needs to retain Python 2.5 compatibility, which does not ship the json
module.