Fix CronTrigger deserialization for JSONSerializer

[tdzienniak]

I noticed that the field _last_fire_time in CronTrigger is not properly deserialized when JSONSerializer is used, which causes the following error:

I added a converter to ensure this field is always a valid datetime.

[coveralls]

coverage: 91.677%. remained the same
when pulling 1fc75f6 on Optibus:master
into 3a8977c on agronholm:master.

[agronholm]

Thanks, good catch! Now I have to fix the same issue in IntervalTrigger and CalendarIntervalTrigger too...