Fix source maps on UMD

Author: bcomnes

webpack.umd.config.babel.js

@@ -14,7 +14,7 @@ const umdConfig = Object.assign({}, baseConfig, {
     libraryTarget: "umd",
     libraryExport: "default"
   },
-  devtool: "hidden-source-map"
+  devtool: "source-map"
 });
 
 umdConfig.plugins.splice(2, 1); // Remove html plugin