Make the remaining ambiguous divisions python 3 compatible.

Refer to #256

puppet/zulip_internal/files/nagios_plugins/check_send_receive_time

@@ -9,6 +9,7 @@ It must be run on a machine that is using the live database for the
 Django ORM.
 """
 from __future__ import print_function
+from __future__ import division
 
 import datetime
 import sys

puppet/zulip_internal/files/nagios_plugins/check_user_zephyr_mirror_liveness

@@ -7,6 +7,7 @@ It must be run on a machine that is using the live database for the
 Django ORM.
 """
 from __future__ import print_function
+from __future__ import division
 
 import datetime
 import os

tools/python-proxy

@@ -82,6 +82,7 @@ Qual a diferen
 
 """
 from __future__ import print_function
+from __future__ import division
 
 import socket, six.moves._thread, select
 PORT = 8085
@@ -193,7 +194,7 @@ class ConnectionHandler(object):
         print('Connected')
 
     def _read_write(self):
-        time_out_max = self.timeout/3
+        time_out_max = self.timeout//3
         socs = [self.client, self.target]
         count = 0
         while True: