Backport PR #6792

---------

**Commit 1:**
Update selenium from 2.48.2 to 2.53.0 to work with latest Firefox 45.0.1

* Original sha: 81821fc
* Authored by LeeDr <lee.drengenberg@elastic.co> on 2016-04-05T20:46:06Z

**Commit 2:**
Update downloadSelenium to match version in intern.js.

* Original sha: 431fe52
* Authored by LeeDr <lee.drengenberg@elastic.co> on 2016-04-06T12:36:06Z

**Commit 3:**
Add selenium-server-standalone jar md5.

* Original sha: 4002140
* Authored by LeeDr <lee.drengenberg@elastic.co> on 2016-04-06T13:46:17Z

tasks/config/downloadSelenium.js

@@ -5,9 +5,9 @@ module.exports = function (grunt) {
   return {
     options: {
       selenium: {
-        filename: 'selenium-server-standalone-2.48.2.jar',
-        server: 'https://selenium-release.storage.googleapis.com/2.48/',
-        md5: 'b2784fc67c149d3c13c83d2108104689',
+        filename: 'selenium-server-standalone-2.53.0.jar',
+        server: 'https://selenium-release.storage.googleapis.com/2.53/',
+        md5: '774efe2d84987fb679f2dea038c2fa32',
         directory: path.join(grunt.config.get('root'), 'selenium')
       }
     }

test/intern.js

@@ -5,7 +5,7 @@ define(function (require) {
   return _.assign({
     debug: true,
     capabilities: {
-      'selenium-version': '2.48.2',
+      'selenium-version': '2.53.0',
       'idle-timeout': 99
     },
     environments: [{