provide a default redis server if not specified

All the tests run successfully on my local machine with this change, however I am not sure how to test this. Also, I can start the server without specifying the redis info.

aarti@local$bin/resque work --queues=default
I, [2013-04-20T19:09:40.220856 #5554]  INFO -- : ["default"]
D, [2013-04-20T19:09:40.220972 #5554] DEBUG -- : resque-2.0.0.pre.1: Starting