title: Gulp error on gulp-ruby-sass task
body: <p>This is the gulpfile in my website project:</p>
  
  <pre><code>var gulp = require('gulp'),
      sass = require('gulp-ruby-sass'),
  
      sass_path = 'src/style/*.scss';
  
  gulp.task('watch', function() {
  
    gulp.watch(sass_path, ['styles']);
  });
  
  gulp.task('styles', function() {
  
    gulp.src(sass_path)
    .pipe(sass())
    .pipe(gulp.dest('dist'));
  });
  
  gulp.task('default', ['styles', 'watch']);
  </code></pre>
  
  <p>When I try to run gulp, it break before finish the task 'styles', and give the output:</p>
  
  <pre><code>[15:37:00] Starting 'styles'...
  [15:37:00] 'styles' errored after 1.92 ms
  [15:37:00] TypeError: Arguments to path.join must be strings
      at path.js:360:15
      at Array.filter (native)
      at Object.exports.join (path.js:358:36)
      at module.exports (/opt/lampp/htdocs/dev/lab/gulp/node_modules/gulp-ruby-sass/index.js:61:15)
      at Gulp.&lt;anonymous&gt; (/opt/lampp/htdocs/dev/lab/gulp/Gulpfile.js:24:9)
      at module.exports (/opt/lampp/htdocs/dev/lab/gulp/node_modules/gulp/node_modules/orchestrator/lib/runTask.js:34:7)
      at Gulp.Orchestrator._runTask (/opt/lampp/htdocs/dev/lab/gulp/node_modules/gulp/node_modules/orchestrator/index.js:273:3)
      at Gulp.Orchestrator._runStep (/opt/lampp/htdocs/dev/lab/gulp/node_modules/gulp/node_modules/orchestrator/index.js:214:10)
      at /opt/lampp/htdocs/dev/lab/gulp/node_modules/gulp/node_modules/orchestrator/index.js:279:18
      at finish (/opt/lampp/htdocs/dev/lab/gulp/node_modules/gulp/node_modules/orchestrator/lib/runTask.js:21:8)
  </code></pre>
  
  <p>How can I fix it?</p>