Fix error in cache detection time

Master job caches were not being cleaned because we were incorrectly detecting
the age of the cache file. To wit:

[DEBUG ] Cache cleaner evaluating jid file at time 2014-05-23 11:53:00
[DEBUG ] Cache cleaner thinks that the current time is: 2014-05-29 
09:23:38.740181
[DEBUG ] Cache cleaner sees the difference as: 5 days, 21:30:38.740181
[DEBUG ] Cache cleaner sees the HOURS difference as: 21.5105555556
[DEBUG ] Cache cleaner is set to only clean jobs after 24 hours

Closes 12396

Please note that in order to backport this fix into previous releases,
this change should be made in master.py.