attempt to prevent pebble to only read SGV values

Try to avoid reporting null for pebble by only considering SGV
records.

Many thanks to @ELUTE for raising flags on this.

lib/pebble.js

@@ -80,7 +80,7 @@ function pebble (req, res) {
         console.error("req.devicestatus.tail", err);
       }
 
-      req.entries.list({count: 2}, get_latest);
+      req.entries.list({count: 2, find: { "sgv": { $exists: true }}}, get_latest);
   });
 }
 function configure (entries, devicestatus) {