Fix leap second representation
Fix leap second representation

The leap second was coded on the wrong unix timestamp. For the
2015-06-30T23:59:60 leap, the stamp share the same unix stamp with
2015-07-01T00:00:00, where I initially use the 23:59:59 stamp. This
representation only affected the unix() method which wrongly returned
the stamp for 23:59:59 instead of 00:00:00.

maximelenoir/hourglass