[backport] Fix unit tests.

The iphone 6 isn't supported anymore by saucelabs, I replaced it by the
iphone 7.

This is a backport of 5b04dfc.

Gruntfile.js

@@ -1,12 +1,11 @@
 /*jshint node: true */
 module.exports = function(grunt) {
+  // see https://saucelabs.com/rest/v1/info/browsers/webdriver
   var browsers = [{
       browserName: "iphone",
-      platform: "OS X 10.8",
-      version: "6"
+      version: "7.0"
   }, {
       browserName: "iphone",
-      platform: "OS X 10.10",
       version: "9.2"
   }, {
       browserName: "android",