Sass not compiled properly on Ubuntu, Windows works fine

Okay so I'm setting up Digital Ocean to work with my project - everything works locally.

When I go and run gulp on my server, sass concats the three input files I give it into app.css, and so version complains that it cannot find the bootstrap.css and font-awesome.css files, giving me a manifest error when visiting my web page.

On windows, sass compiles into three separate files so everything works properly!

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

elixir(function (mix) {
    mix.sass([
        'bootstrap.scss',
        'font-awesome.scss',
        'app.scss'
    ]);

    mix.scripts([
        'jquery.js',
        'jqueryui.js',
        'bootstrap.js',
        'jquery.payment.js',
        'custom.js'
    ]);

    mix.version([
        "css/app.css",
        "css/font-awesome.css",
        "css/bootstrap.css",
        "js/all.js",
        "images/logo.png",
        "images/logo-facture.png",
        "images/favicon.png",
        "images/jumbotron.jpg"
    ]);

    //mix.phpUnit();
});

What am I doing wrong...? Especially since everything works properly on my local machine.