Use alternative to rack-jekyll to run accessibility tests

[phrdang]

See commit message here: e1efa4e

See also #17

[cycomachead]

In another non-Jekyll static app I used Rack::File directly. It was a fairly simple change, but rack::file is also throwing future deprecation warnings.

We do need a hook to ensure the Jekyll site is fully built before starting the test run. We could in CI just call Jekyll build and have a dev warning? I mean it's not as nice but in terms of workability it's not a huge deal.

I haven't looked at what updating rack-Jekyll would be but that honestly shouldn't be a huge dependency.

[cycomachead]

OK, well Rack::File became Rack::Files so that wasn't too hard of a fix, but I do feel like the path mapping is a little awkward...oh well.

See #37 which was meant to just be styling but then I realized none of the CI tests were actually loading the right CSS so I went down the path of just refactoring a bunch.