fix graph not being updated with prediction

the fix in fd8d2dd didn't work as the prediction loads before the
vehicles are loaded in. So, we initiate the prediction fetch after
we've loaded all the vehicles.

Author: rossengeorgiev

js/tracker.js

@@ -2268,8 +2268,6 @@ function refresh() {
     complete: function(request, textStatus) {
         clearTimeout(periodical);
         periodical = setTimeout(refresh, timer_seconds * 1000);
-
-        if(periodical_predictions === null) refreshPredictions();
     }
   });
 }
@@ -2737,6 +2735,8 @@ function update(response) {
               zoom_on_payload();
           }
 
+          if(periodical_predictions === null) refreshPredictions();
+
           ajax_inprogress = false;
         }
     };