Fix API request, upgrade Ruby, Rails & Unicorn

This fixes the site by updating the version parameter in requests to Foursquare API. The previous version was no longer supported as of July 1st.

It also upgrades the app to Ruby 2.1.3, latest Rails 3.2.x, and switches the server from Thin to Unicorn. In production it's configured to spin up 4 unicorn workers which makes the app more performant under traffic.

I've taken the liberty of deploying these fixes to Heroku already since the site was down.