configure Rails to create non-digest versions of vendored assets

Rails in v4 has stopped supporting non-digest assets in production. This would require any vendored assets to be managed through the asset pipeline as a gem. (See rails/sprockets-rails#49 for more information.) There are a few different ways to get around this, but using https://github.com/alexspeller/non-stupid-digest-assets seems to be the simplest, sanest way to go.