convert from Sinatra/Heroku to Jekyll/gh-pages

Heroku is sunsetting their 'legacy' IP address custom domain configurations. More info here: https://devcenter.heroku.com/articles/moving-to-the-current-routing-stack

This means we would have to point the `opencityapps.org` domain to `open-city.herokuapp.com` as a `CNAME` record instead of pointing to a set of IP addresses with `A` records.

This is all good and well for most of our apps, but for this one we have email addresses associated with the domain, which using the `CNAME` method breaks: http://serverfault.com/questions/91712/dns-using-cnames-breaks-mx-records

gh-pages supports IP address DNS records so we can avoid this issue by switching.

We also get the added bonus of Jekyll serving up static html files, reducing the technical overhead of what is essentially just a one page static website.