Breaking crontab specification for ranges

The specification listed in crontab(5) man page states:


> A field may be an asterisk (*), which always stands for ``first-last''.
> ...
> Ranges can include "steps", so "1-9/2" is the same as "1,3,5,7,9".

The current version has 1-9/2 generate 2,4,6,8, when it should generate the list stated above.
The current version has 4-13/3 generate 6,9,12, when it should generate 4,7,10,13.

The fix removed the mod method and replace it with a step in xrange.