The Gemfile expects that you're running ruby version 2.1.2. If you have installed rbenv, you should be able to do rbenv install 2.1.2. Alternatively, you should be okay to comment out the ruby line in Gemfile and use whatever ruby you have installed (though please don't check in that change).`
Clone down and cd into the project and then run
$ bundle
$ bundle exec rake db:setup
$ bundle exec rails s
You should then be able to navigate to localhost:3000 and see the welcome page. Press Ctrl-C in the terminal running the server to stop it.
You can manipulate sample Topics in the rails console as follows:
$ bundle exec rails console
> t = Topic.new
> t.title = '...'
> t.description = '...'
> t.save!
> exit