Heroku Wiki Information could be changed: websockets is now enabled on default

I just followed [this](https://github.com/fzaninotto/uptime/wiki/Deploy-to-Heroku)
and noticed that 
` heroku labs:enable websockets`
returns
`Couldn't find that feature.`
because [websockets are now enabled by default](https://stackoverflow.com/questions/25012829/enabling-websockets-on-heroku). so this line could be removed.