Bump minimum required version for simplejson to 2.2.0

oslo-incubator commit ad248f6 adds usage of the namedtuple_as_object
kwarg to simplejson's json.dumps.

oslo-incubator's test-requirements requires simplejson >= 2.0.9 but
namedtuple_as_object wasn't available until 2.2.0.

This is important for anyone doing keystone database migrations on RHEL
6.5 with simplejson since the migrations will fail if you don't have a
new enough simplejson (keystone is using this code now).

Change-Id: I59f4e9c42ad83a305d810e52e440ff51ff5b7f0e
Closes-Bug: #1361230