Gemfile: pin mime-types version 1

mime-types 2 requires Ruby version >= 1.9.
Gemfile: use capybara in Ruby >= 1.9.3

Gem::InstallError: nokogiri requires Ruby version >= 1.9.2.
An error occurred while installing nokogiri (1.6.1), and Bundler cannot continue.
Make sure that `gem install nokogiri -v '1.6.1'` succeeds before bundling.