Preserve existing requirejs configuration

Our requirejs setup is now a little more considerate when establishing requirejs configuration i.e. it now considers any existing configuration.

This is necessary when requiring to pass configuration from the html template e.g.:

    <script type="text/javascript">
        var require = {
            config: {
                'controllers/Application' : {
                    tzs: '@routes.Application.tzs()'
                }
            }
        };
    </script>

Prior to the fix, we were simply overwriting any existing configuration.