require rbconfig to work around an issue with `--watch`:

run `bundle` to update wdm
```
>jekyll serve --watch
Configuration file: C:/workspace/jekyllcompass/_config.yml
            Source: C:/workspace/jekyllcompass
       Destination: C:/workspace/jekyllcompass/_site
      Generating...    create ../_site/css/print.css
   create ../_site/css/screen.css
done.
 Auto-regeneration: enabled
C:/Ruby193/lib/ruby/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:45:in `require': cannot load such file -- wdm (LoadError)
        from C:/Ruby193/lib/ruby/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:45:in `require'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/listen-1.3.1/lib/listen/adapter.rb:207:in `load_dependent_adapter'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/listen-1.3.1/lib/listen/adapters/windows.rb:33:in `load_dependent_adapter'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/listen-1.3.1/lib/listen/adapter.rb:198:in `usable?'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/listen-1.3.1/lib/listen/adapters/windows.rb:25:in `usable?'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/listen-1.3.1/lib/listen/adapter.rb:190:in `usable_and_works?'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/listen-1.3.1/lib/listen/adapter.rb:57:in `block in select_and_initialize'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/listen-1.3.1/lib/listen/adapter.rb:55:in `each'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/listen-1.3.1/lib/listen/adapter.rb:55:in `select_and_initialize'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/listen-1.3.1/lib/listen/listener.rb:291:in `initialize_adapter'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/listen-1.3.1/lib/listen/listener.rb:283:in `setup'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/listen-1.3.1/lib/listen/listener.rb:52:in `start'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/jekyll-1.3.0/lib/jekyll/commands/build.rb:56:in `watch'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/jekyll-1.3.0/lib/jekyll/commands/build.rb:8:in `process'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/jekyll-1.3.0/bin/jekyll:97:in `block (2 levels) in <top (required)>'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/commander-4.1.5/lib/commander/command.rb:180:in `call'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/commander-4.1.5/lib/commander/command.rb:180:in `call'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/commander-4.1.5/lib/commander/command.rb:155:in `run'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/commander-4.1.5/lib/commander/runner.rb:402:in `run_active_command'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/commander-4.1.5/lib/commander/runner.rb:66:in `run!'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/commander-4.1.5/lib/commander/delegates.rb:11:in `run!'
        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/commander-4.1.5/lib/commander/import.rb:10:in `block in <top (required)>'
```