Get this example app ready for heroku-18 stack

[klausmeyer]

Hi 👋,

today I've tried to create a new Heroku app from this example app with the "Deploy to Heroku" button.

Unfortunately the used ruby version seems to be not supported on the heroku-18 stack so I thought it might be worth a PR to fix this.

What I did:

• Use latest ruby version (2.5.3)
• Upgrade Sinatra to 2.x
• Harmonize string quoting-style in web.rb

Cheers, Klaus

//cc @neilmiddleton, @hone Maybe one of you could have a look please? Thank you 🙇🏻‍♂️