Make long-running jobs not skip periods

[SijmenHuizenga]

As described in #304, jobs started on one day, running for so long they finish in the next day, the job wasn't being scheduled in the day the job finished, but the day after. The problem:

• Job scheduled to run every day at 23:00
• Job starts on day 1 at 23:00 and finishes the next day (day 2) at 00:03
• The actual behaviour (before this change) is that the job is scheduled to run at day 3 on 23:00
  The expected behaviour (after this change) is that after finishing, the job is scheduled to run at day 2 at 23:00

[coveralls]

Coverage remained the same at 100.0% when pulling 8228daf on SijmenHuizenga:patch-304 into 5033fbd on dbader:master.