Need to check that Elixir is ready before merging webpack config

[AbstractEverything]

When merging some additional loaders with Elixir.webpack.mergeConfig() in my gulpfile.js I encountered an error: "TypeError: Cannot read property 'mergeConfig' of undefined"

I was able to solve this by checking that Elixir was ready before merging the additional config:

Elixir.ready(function() {
    Elixir.webpack.mergeConfig({
        // ...
    });
});

Perhaps this should be noted in the readme?