39.js

var watch = require('gulp-watch');

// ...

gulp.task('watch', function() {
  // rebuild when jsx, js, html change, in src directory, including subdirectories
  watch('./src/**/*.{jsx,js,html}', function() {
    gulp.start('build');
  });
});

// already existing "server" task

gulp.task('server', ['build'], function() {
  gulp.start('watch'); // 1 LINE ADDED FOR "server" TASK
  gulp
  .src('./dist') // everything from ./dist directory
  .pipe(server({ // pipe to server object
    livereload: true, // reload when files are changed
    open: true // and open the browser
  }));
});