Update usage instructions for Webpack 2.
Update usage instructions for Webpack 2.

There is now a Webpack 1 [deprecation warning](http://webpack.github.io/docs/using-loaders.html) at the top of the old link and not using the full `elm-webpack-loader` name throws an error.

```
BREAKING CHANGE: It's no longer allowed to omit the '-loader' suffix when using loaders.
                 You need to specify 'elm-webpack-loader' instead of 'elm-webpack',
                 see https://webpack.js.org/guides/migrating/#automatic-loader-module-name-extension-removed
```
rtfeldman/elm-webpack-loader,rtfeldman/elm-webpack-loader