Fix last_reachable timestamp bug

Change-Id: I0d479c9aef59866db164790247eca2c3c6e1f0bf
diff --git a/app.py b/app.py
index 44c91ec..e7ede6a 100644
--- a/app.py
+++ b/app.py
@@ -78,9 +78,10 @@
                       default="3333")
     return parser
 
-def update(roc, prom, devices):
-    roc.update_devices(devices)
-    prom.update_devices(devices)
+def update(roc, prom, old):
+    new = roc.update_devices(old)
+    prom.update_devices(new)
+    return new
 
 def probe(devices):
     for imsi_id, device in devices.items():
@@ -97,11 +98,11 @@
 def work_thread(roc, prom):
     global devices, lock
     while True:
-        temp_devices = {}
-        update(roc, prom, temp_devices)
-        probe(temp_devices)
+        new = update(roc, prom, devices)
+        probe(new)
         with lock:
-            devices = temp_devices
+            devices = new
+
         time.sleep(5)
 
 if __name__ == '__main__':