Added babel in karma config

Author: fayyazarshad

packages/optimizely-sdk/karma.base.conf.js

@@ -24,6 +24,14 @@ module.exports = {
 
   webpack: {
     mode: 'production',
+    module: {
+      loaders: [
+        {
+          test: /\.js$/,
+          loader: 'babel-loader?presets[]=es2015'
+        }
+      ]
+    }
   },
 
   //browserStack setup