Start selenium-standalone via wdio.conf.js

wdio.conf.js

/*global browser:false*/
var selenium = require("selenium-standalone");
var Promise = require("bluebird");

exports.config = {
  // Other configs go here

  //
  // =====
  // Hooks
  // =====
  // Run functions before or after the test. If one of them returns with a promise, WebdriverIO
  // will wait until that promise got resolved to continue.
  //
  // Gets executed before all workers get launched.
  onPrepare: function() {
    Promise.promisifyAll(selenium);

    var promise = selenium.installAsync({
      logger: function(message) {
        console.log(message);
      }
    }).then(function() {
      console.log("Starting Selenium");

      return selenium.startAsync().then(function(child) {
        selenium.child = child;
        console.log("Selenium started");
      });
    });

    return promise;
  },
  //
  // Gets executed after all workers got shut down and the process is about to exit. It is not
  // possible to defer the end of the process using a promise.
  onComplete: function() {
    selenium.child.kill();
    console.log("\nSelenium process ended. Test suite complete");
  }
};