fix CronLine#previous_time rdoc
Fix stack overflow

This fixes gh-90

Running the newly added spec line used to cause a stack overflow due to
the ineficient way CronLine#previous_time was implemented. Now the
implementation is a mirror of next_time
Merge pull request #91 from yassenb/fix-stack-overflow

fix CronLine#previous_time stack overflow
add Yassen to the contributor list