updates(selenium): update chromedriver and selenium-standalone

Selenium-standalone update to 2.51.0. Update chromedriver to 2.21.

Chromedriver changelog: http://chromedriver.storage.googleapis.com/2.20/notes.txt
Selenium changelog: https://github.com/SeleniumHQ/selenium/blob/master/dotnet/CHANGELOG

Author: juliemr

config.json

@@ -1,7 +1,7 @@
 {
   "webdriverVersions": {
-    "selenium": "2.48.2",
-    "chromedriver": "2.20",
-    "iedriver": "2.48.0"
+    "selenium": "2.51.0",
+    "chromedriver": "2.21",
+    "iedriver": "2.51.0"
   }
 }