add default pickle cache version

Change-Id: I3efcd68425a79d84241d4f7c16ba385127331deb
Signed-off-by: cloudnull <kevin@cloudnull.com>

monitorstack/utils/__init__.py

@@ -123,7 +123,10 @@ class LocalCache(object):
 
         :returns: object
         """
-        return diskcache.Cache(directory=self.cache_path)
+        return diskcache.Cache(
+            directory=self.cache_path,
+            disk_pickle_protocol=2
+        )
 
     def lc_open(self):
         """Open shelved data.