Extract requirejs callback definition to an independent function.

I have a simple problem: I want to make angularjs depends on jquery using the webjars generated requirejs config.

Using the current webjar-locator config generation, I have something like this in my play template:
```html
<script>
@Html(org.webjars.RequireJS.getSetupJavaScript(controllers.routes.WebJarAssets.at("").url))
  var __require_callback = require.callback;
  require.callback = function() {
    __require_callback();
    requirejs.config({
      shim: {
        "angular":{"exports":"angular", "deps": ["jquery"]}
      }
    });
  }
</script>
```

I have to save the generated callback to be able to callit and override the requirejs config with my new angular dependency.

I just propose to extract the current callback in a specific javascript function and generate a basic requirejs config that uses this callback.

Using this PR, my template code should looks like: 
```html
<script> 
@Html(org.webjars.RequireJS.getSetupJavaScript(controllers.routes.WebJarAssets.at("").url))
  require.callback = function() {
    webjars.requirejs_callback();
    requirejs.config({
      shim: {
        "angular":{"exports":"angular", "deps": ["jquery"]}
      }
    });
  }
</script>
```