chore(deps): bump chromedriver and iedriver versions

IEDriver from 2.47.0 to 2.48.0

ChromeDriver from 2.19 to 2.20. Changelog:
http://chromedriver.storage.googleapis.com/2.20/notes.txt

Author: juliemr

config.json

@@ -1,7 +1,7 @@
 {
   "webdriverVersions": {
     "selenium": "2.48.2",
-    "chromedriver": "2.19",
-    "iedriver": "2.47.0"
+    "chromedriver": "2.20",
+    "iedriver": "2.48.0"
   }
 }