Update the version number (on master) from "4.3.0" to "5.15.0", becau…

…se I just can't stand it being that out of date any longer.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "collaborative-learning",
-  "version": "4.3.0",
+  "version": "5.15.0",
   "description": "Collaborative Learning environment",
   "main": "index.js",
   "engines": {