Use MultiJSON for JSON parsing

By specifying a runtime dependency on `multi_json` instead of the `json` gem, it gives more flexibility to the end-user to use whatever JSON library they prefer (falling back to `json` library on Ruby 1.9 and `okjson` on Ruby 1.8). This also allows greater compatibility across interpreters, which may not support C extensions.

While I was updating this dependency, I also swapped in `kramdown` for `rdiscount` (which also depends on C extensions) and updated the `aruba` and `rspec` development dependencies to allow newer versions. Finally, I alphabetized the dependencies in the gemspec within their respective groups.