Use timezone info for success and failure cases

Nearly identical to https://github.com/ilyakatz/resque-history/pull/32. During my hasty web edit to add timezone info to job failures I failed to see the same format string being used about 10 lines below for successful jobs :flushed: 

If you could merge this an cut a gem release with the changes I'd be very appreciative. My apologies for missing the second format string the first time around.