Possible fix for #4: lookup root by original path

As discussed in #4.

I did have to relax the dependencies to the two digit precision to get a recent simple-navigation and to work with the latest middleman branches. More explanation at https://github.com/middleman/middleman/issues/501#issuecomment-43938278.

I also have one failing test, but it fails in my environment with the original code as well. Hopefully you can figure out what's causing this:
```
    Then the Subpage menu item should not be selected # features/step_definitions/nav_steps.rb:1
      expected "HomeAnotherChildSubpage" not to match "Subpage" (RSpec::Expectations::ExpectationNotMetError)
      features/basic.feature:14:in `Then the Subpage menu item should not be selected'
```