session: simple cache fix

* Fixes issue with simple in-memory cache not being able to
  work with timedelta timeouts.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
installation: relax requirement on reportlab

* Relax constraint on reportlab to allow 2.x release series since
  Python 2.6 support was removed in reportlab 3.x series.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>