fix two bugs in the code and two bugs in the tests

* set_time = lock_value.to_i, so it will never be not true, use
  lock_value in the conditional
* We want set_time to be less than current time - ttl, not more than
* make sure to delete old locks in setup
* if we want a lock to be expired it should be pushed into the past, not
  the future
Merge pull request #7 from dougcole/fix_bugz

fix two bugs in the code and two bugs in the tests