fix(umd): now correctly use production string for UMD bundles

At the time of fixing this...

```sh
yarn dev-utils libsize

The gizipped UMD bundle sizes are:
 - dist/umd/react-md.production.min.js 82.51 KB (- 1.63 KB)
 - dist/umd/react-md-with-font-icons.production.min.js 106.63 KB (- 1.53 KB)
 - dist/umd/react-md-with-svg-icons.production.min.js 172.64 KB (- 1.59 KB)
```

Author: mlaursen

rollup.config.js

@@ -54,7 +54,7 @@ function createConfig(type) {
       resolve(),
       commonjs(),
       replace({
-        'process.env.NODE_ENV': JSON.stringify(process.env.NODE_ENV),
+        'process.env.NODE_ENV': JSON.stringify('production'),
       }),
     ],
   };