Improve `bin/setup` script for installing bundler and gems
Improve `bin/setup` script for installing bundler and gems

This installs first Bundler then installs dependencies listed in the
Gemfile only if needed

Rails default `bin/setup` script does the same:
https://github.com/rails/rails/blob/v4.2.0/railties/lib/rails/generators/rails/app/templates/bin/setup#L11

craftsmen/roll,craftsmen/roll,craftsmen/roll