Added gem install bundler within the instructions

Recently at RailsBridge Tulsa's first class we ran into an issue where
bundler was not installed. The students on had to first run `gem
install bundler` in order to run `bundle install --without production`.
This addition should help prevent problems in the future.

Author: nearbycoder

sites/en/installfest/deploy_a_rails_app.step

@@ -87,6 +87,7 @@ step "Deploy your app to Heroku" do
     end
 
     console <<-BASH
+gem install bundler
 bundle install --without production
     BASH