fix the noisy spray logging issue

Without this fix, job server generates the following annoying log every 12 seconds which pollute the log file. 
[2014-11-14 21:15:36,760] WARN  erver.HttpServerConnection [] [akka://JobServer/user/IO-HTTP/listener-0/164882] - Connection is already closed, dropping command ConfirmedClose

The fix is to upgrade the Spray version. See details here:  
https://groups.google.com/forum/#!msg/spray-user/YN2ocRzwhY0/KJOegaDIep8J