Upgrade celluloid gem to avoid a dependency issue

In the old version, celluloid didn’t lock the timers gem down to a major
  version (it used >=), so the most recent version with a different API 
  was fetched instead.

As far as I can tell, the only thing that changed is the way celluloid
  handles making a method asynchronous.  I’m not sure if woodhouse was following
  a similar async naming convention (! indicates async) and if so, whether
  changes are desired to bring the woodhouse api into the new convention.