Task 'default' is not in your gulpfile

I'm trying to build a very simple elixir gulp file but am running into problems right from the start. I've installed everything with npm (npm install --no-bin-links), I have the gulp version 3.9.1 (as shown with gulp -v) and my gulp file looks like:

var elixir = require('laravel-elixir');

elixir(function(mix) {
    mix.copy('node_modules/font-awesome/fonts', 'public/assets/fonts');

    mix.sass(
        'app.scss',
        'public/assets/css/site.css',
        {
            includePaths: [
                'node_modules/foundation-sites/scss',
                'node_modules/font-awesome/scss'
            ]
        }
    );

    mix.babel(
        [
            'vendor/jquery.js',
            'vendor/fastclick.js'
        ],
        'public/assets/js/site.js',
        'node_modules/foundation-sites/js/'
    );

    mix.version([
        'assets/js/site.js',
        'assets/css/site.css'
    ]);
});

So really nothing out of the ordinary. However, when I run gulp on the command line I just get back the message:

[08:58:42] Using gulpfile D:\git\laravel\initial\gulpfile.js
[08:58:42] Task 'default' is not in your gulpfile
[08:58:42] Please check the documentation for proper gulpfile formatting

Where am I going wrong? Any help is appreciated!