clear_expired_stashes was broken because expire_at is inside the hash content

Sensu stash returns a hash that looks like this: 

{"path"=>"silence/dev-z2/ES-Cluster-Health", "content"=>{"description"=>"You must expired", "owner"=>"zuhaib@hipchat.com", "timestamp"=>1374184850, "expire_at"=>"2013-07-18T22:15:00+00:00"}}

So for the clear stash to work you must access ['content']['expire_at'].  Currently master is broken.

Tested on sensu 0.10.0