autocomplete.js fix tag duplicate filtering

Apparently returning the result of an indexOf in the filter callback
doesn't correctly evaluate to true or false... This seems to work
better.

browser/js/autocomplete.js

@@ -209,7 +209,7 @@ VidjilAutoComplete.prototype = {
             if (res < 0) {
                 seen.push(elem.name);
             }
-            return res;
+            return res < 0;
         })
 
         $input.atwho('load', at, loaded_data);