SQS Transport: Fix for KeyError on message acknowledgment-- (#73)

Avoids duplicate keys on message delivery tags by using UUIds instead
of a simple counter.

Closes #73

kombu/transport/SQS.py

@@ -285,6 +285,9 @@ class Channel(virtual.Channel):
                    aws_secret_access_key=conninfo.password,
                    port=conninfo.port)
 
+    def _next_delivery_tag(self):
+        return uuid()  # See #73
+
     @property
     def sqs(self):
         if self._sqs is None: