Add test to reproduce race condition in update_schedule method
FIX: typo in string interpolation

Typo in string interpolation + renaming the test as suggested by
@meatballhat.
FIX: prevent race conditions in update_schedule

As noted by @rychoo2 in issue #230, the memoizing of Resque.schedule was
causing a race condition. The easy solution was to refresh the hash of
all the schedueles for each of the job handled in the update_schedule
cycle.
Merge pull request #397 from resque/update_schedule-race-condition

update_schedule race condition