Fixed typo in util.js,

resulted from moving is_cell_rendered into util.js

IPython/html/tests/util.js

@@ -417,10 +417,10 @@ casper.cell_has_class = function(index, classes) {
     }, {i : index, c: classes});
 };
 
-casper.is_cell_rendered = function (index, msg) {
+casper.is_cell_rendered = function (index) {
     return this.evaluate(function(i) {
         return !!IPython.notebook.get_cell(i).rendered;
-    }, {i:i});
+    }, {i:index});
 };
 
 casper.assert_colors_equal = function (hex_color, local_color, msg) {