Update ruby.md

Trying to install the gem bundle without bundler fails with:
/usr/local/lib/ruby/2.0.0/rubygems/dependency.rb:296:in `to_specs': Could not find 'bundler' (>= 0) among 8 total gem(s) (Gem::LoadError)
        from /usr/local/lib/ruby/2.0.0/rubygems/dependency.rb:307:in `to_spec'
        from /usr/local/lib/ruby/2.0.0/rubygems/core_ext/kernel_gem.rb:47:in `gem'
        from /usr/local/bin/bundle:22:in `<main>'

Installing the bundler (with sudo gem install bundler) at the end of Step 4 fixes this.