writeFile deprecation warning

[godong9]

Hi. Thank you for your project.

I found below message when pm2-gui running. (node.js version 8.x.x)

[ERROR] (node:133073) [DEP0013] DeprecationWarning: Calling an asynchronous function without callback is deprecated.

And error occured when node.js version 10.13.0.

TypeError [ERR_INVALID_CALLBACK]: Callback must be a function
    at maybeCallback (fs.js:129:9)
    at Object.writeFile (fs.js:1156:14)
    at Object.fork (~/.nvm/versions/node/v10.13.0/lib/node_modules/pm2-gui/lib/daemon.js:121:12)
    at start (~/.nvm/versions/node/v10.13.0/lib/node_modules/pm2-gui/lib/daemon.js:47:30)
    at ~/.nvm/versions/node/v10.13.0/lib/node_modules/pm2-gui/node_modules/async/lib/async.js:718:13
    at Immediate.iterate (~/.nvm/versions/node/v10.13.0/lib/node_modules/pm2-gui/node_modules/async/lib/async.js:262:13)
    at runCallback (timers.js:705:18)
    at tryOnImmediate (timers.js:676:5)
    at processImmediate (timers.js:658:5)