Fix locked? method to use Redis exists command instead of exist, whic…

…h doesn't appear to be implemented by Redis.

lib/resque/plugins/lock.rb

@@ -50,7 +50,7 @@ def lock(*args)
 
       # Convenience method, not used internally.
       def locked?(*args)
-        Resque.redis.exist(lock(*args))
+        Resque.redis.exists(lock(*args))
       end
 
       # Where the magic happens.