Fix 59 seconds

[joshuathayer]

Hi! I found a bug while using your library-

(next-date (t/date-time 2015 04 21 06 00 59 000) "0 * * * * *")

incorrectly yields

#<DateTime 2015-04-21T06:00:00.000Z>

rather than the correct

#<DateTime 2015-04-21T06:01:00.000Z>

This PR fixes that bug, and adds a test to demonstrate. I also added some tests around other overflow behavior.

clj-time gets a version bump in order to use the relatively new t/equal?

Thanks!

[mustafashabib]

👍