demo: select codepen preview is not working

[devversion]

When clicking on the codepen button for the first select demo, the demo won't load properly inside of CodePen.

• https://material.angularjs.org/HEAD/demo/select

This is caused by the example directive usage.

The <code><md-select></code> component can be used within a

This will be interpreted as a normal md-select directive, without any ng-model.

If the developer isn't specifying any ng-model on the md-select we throw an error.

Controller 'ngModel', required by directive 'mdSelect', can't be found!

cc. @topherfangio