Swagger ui from dist

This is on top of https://github.com/BrandyMint/grape-swagger-rails/pull/30, so look at that one first.

Added a Rake task to update Swagger UI from [dist](https://github.com/wordnik/swagger-ui), by running `bundle exec rake swagger_ui:dist:update`. This will clone the swagger-ui repo, copy files around and edit anything that needs to be edited. There're some tricky things arounds images or included JS, the task will warn you that new JS or CSS has been added or removed so you can update the task - this has to be done manually because the order of these things matters.