Proxy URL not working with BrowserSync Gulp task

I had the same problem and did the following to stop browser-sync from using the default adress/port:

gulp.task('bs', function () {
    browserSync.init(null, {
        proxy: 'localhost:8080', // 'dev.site.com' in your example
        port: 5000
    });
});

This worked for me and change the browser-scyn adress to localhost:5000