[fixed] Updates webpack distribution config to reference the correct …

…externals (reactjs#210)

Currently, if not using any kind of module loader, webpack falls back to looking for dependencies on the root (e.g. `window` in the browser). React and ReactDOM both have different names when looking up on the root, and these different names can be specified by making the `externals` prop an object where the values represent an object of loader type => name.

See [react-redux's webpack config](https://github.com/reactjs/react-redux/blob/master/webpack.config.js) for an example.

webpack.dist.config.js

@@ -2,17 +2,30 @@ var webpack = require('webpack');
 var UglifyJsPlugin = webpack.optimize.UglifyJsPlugin;
 var env = process.env.WEBPACK_ENV;
 
+var reactExternal = {
+  root: 'React',
+  commonjs2: 'react',
+  commonjs: 'react',
+  amd: 'react'
+};
+var reactDOMExternal = {
+  root: 'ReactDOM',
+  commonjs2: 'react-dom',
+  commonjs: 'react-dom',
+  amd: 'react-dom'
+};
+
 module.exports = {
 
   entry: {
     'react-modal': './lib/index.js',
     'react-modal.min': './lib/index.js'
   },
 
-  externals: [
-    'react',
-    'react-dom'
-  ],
+  externals: {
+    'react': reactExternal,
+    'react-dom': reactDOMExternal
+  },
 
   output: {
     filename: '[name].js',