Enforce that geolocation.timestamp is an integer (close snowplow#602)

Author: mhadam

src/js/tracker.js

@@ -1341,7 +1341,7 @@
 							altitudeAccuracy: coords.altitudeAccuracy,
 							bearing: coords.heading,
 							speed: coords.speed,
-							timestamp: position.timestamp
+							timestamp: Math.round(position.timestamp)
 						}
 					};
 					commonContexts.push(geolocationContext);