Adjust to new roc api

Change-Id: I67dc8d45ec6421d957af697b2f650c66b7c73a3d
diff --git a/device.py b/device.py
index 963d5ce..60db1aa 100644
--- a/device.py
+++ b/device.py
@@ -15,3 +15,6 @@
         self.ip = ip
         self.reachable = False
         self.last_reachable = last_reachable
+
+    def __str__(self):
+        return("imsi_id={}, imsi={}, ip={}".format(self.imsi_id, self.imsi, self.ip))