The `:ruby_18` platform in Bundler does not include JRuby in 1.8 mode (only MRI 1.8 and Rubinius in 1.8 mode). There exists a separate `:jruby` platform, but there’s no way to differentiate between 1.8 and 1.9 mode. If we really want to test on JRuby in 1.8 mode, we can specify the old version of `mime-types` for all `:jruby` platforms.