gulpfile.js

var gulp = require('gulp');
var sass = require('gulp-ruby-sass');
var browserSync = require('browser-sync');
var reload = browserSync.reload;

gulp.task('sass', function() {
  return sass('app/scss/main.scss')
    .pipe(gulp.dest('app/css'))
    .pipe(reload({ stream:true }));
});

gulp.task('serve', ['sass'], function() {
  browserSync({
    server: {
      baseDir: 'app'
    }
  });

  gulp.watch('app/scss/*.scss', ['sass']);
  gulp.watch("app/index.html").on('change', browserSync.reload);
});

package.json

{
  "name": "frontend-template",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "dependencies": {
    "browser-sync": "^2.18.13"
  },
  "devDependencies": {
    "gulp": "^3.9.1",
    "gulp-ruby-sass": "^2.1.1"
  },
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

gulp works with sass-3.5.5