Upgraded included simplejson to 2.0.7.

Also changed importing logic to prefer a system-installed version of
simplejson (unless it's an earlier version that does not contian the C
speedups), then the json module from Python 2.6, then the version
shipped with Django.

Fixed #9266.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9707 bcc190cf-cafb-0310-a4f2-bffc1f526a37