Change URLs to match our guidelines

`/designprinciples` becomes `/design-principles`
`/designprinciples/styleguide` becomes `/design-principles/style-guide`

This needs to be done for 2 reasons:

1. It's the correct thing to do.
2. The content path currently overlaps with the assets prefix.  This is causing problems with decoupling the asset serving from the static application.  Additionally future versions of Rails will refuse to work with this overlap.

I also added a `.ruby-version` because this didn't have one.