Fix last_reachable timestamp bug

Change-Id: I0d479c9aef59866db164790247eca2c3c6e1f0bf
diff --git a/device.py b/device.py
index 2b1ffcd..963d5ce 100644
--- a/device.py
+++ b/device.py
@@ -8,10 +8,10 @@
 
 
 class Device(object):
-    def __init__(self, imsi_id, imsi, ip=None):
+    def __init__(self, imsi_id, imsi, last_reachable=datetime.min, ip=None):
         # log.debug("creating device, imsi_id={}, imsi={}, ip={}".format(imsi_id, imsi, ip))
         self.imsi_id = imsi_id
         self.imsi = imsi
         self.ip = ip
         self.reachable = False
-        self.last_reachable = datetime.min
+        self.last_reachable = last_reachable