esri-loaderify JSAPI demo

[tomwayson]

Do what i suggested in https://github.com/Esri/arcgis-rest-js/pull/221#issuecomment-397389485

Here's why:

• what that demo currently shows (loading the JSAPI first, then using it to load arcgis-rest-js libs) is an edge case
• we have a guide for AMD that shows how to do that
• esri-loader works in es5 so it's a very small change, see https://github.com/Esri/arcgis-rest-js/commit/28dc49b3315e61766c7820c77c7af82b5fa06b3a

I will also update the README to refer to the aforementioned guide for anyone that actually needs to load arcgis-rest-js via AMD.

[jgravois]

sounds good to me.

[jgravois]

if no further action is taken, @tomwayson's self promotional issue will be closed manually on 6/19/2019.