First get costart:

curl -L -o costart https://gist.github.com/raw/4684183/d65e44277cf4aab221c27b6987d2038d9e198f5a/costart
chmod +x costart

Save it somewhere in your $PATH for convenience.

Then:

costart dir
cd dir
bundle install # might be optional
bundle exec rackup

Open http://localhost:9292/