Upgrade Swagger UI and use new standalone version

[duffn]

This could use the simplified HTML/JS/CSS version of Swagger UI.

• Migrate to this version https://github.com/swagger-api/swagger-ui/blob/master/docs/usage/installation.md#plain-old-htmlcssjs-standalone
  • This is an example only. Need to pull down the assets into the repo.
• Update Rake task to pull new versions as necessary
• Update Swagger index.html.erb as necessary

I'd also like to:

• [x] Update the dummy app in this style of 6.0+ if not already. See separrate issue: https://github.com/ruby-grape/grape-swagger-rails/issues/109
• [x] Add a test matrix for newer versions of grape-swagger, Ruby, and Rails. See https://github.com/ruby-grape/grape-swagger-rails/pull/107

I'm trying to figure out the best order to do all of these.

[dblock]

This project needs some love. You probably know best in which order to start with. I typically begin CI, so would do the "also like to" part first.

[duffn]

Agreed, here's a first pass at the test matrix: https://github.com/ruby-grape/grape-swagger-rails/pull/107