Try to get ping-centre into the babel chain

- Remove the restriction on ES6 module processing from babel config
  (hopefully this is the right option to change)

- Rather than exclude all of node_modules, just include the one module
  we need to process, and implicitly exclude the rest

  - `include` syntax based on webpack/webpack#2031 (comment)

addon/src/webpack.config.js

@@ -21,11 +21,13 @@ const config = {
         test: /\.js$/,
         loader: 'babel-loader',
         query: {
-          presets: [ [ 'es2015', { modules: false } ], 'stage-2' ],
+          presets: [ 'es2015', 'stage-2' ],
           plugins: [ 'transform-flow-strip-types' ]
         },
-        exclude: /node_modules/,
-        include: __dirname
+        include: [
+          __dirname,
+          /node_modules\/ping-centre/
+        ]
       }
     ]
   },