Keeping the build files current and adding deep links to the Intializr.

There we get a 404, not a 405

So to enable and expose it we need this 2 lines:
management.endpoint.shutdown.enabled=true
management.endpoints.web.exposure.include=health,info,shutdown

Updated the Spring Boot version and the Gradle and Maven wrappers.

Also removed the Initializr image and updated the README to match.

Previous URL (http://groovy.codehaus.org/Grape) doesn't work.
Changed to new domain (www.groovy-lang.org).

I rewrote the document and edited the code (mostly changing package names) to use the Spring Initializr

I also upgraded the build files to Spring Boot 2.2.0, which necessitated changing the tests to use JUnit 5 and replacing the gradle wrappers.

Resolves gh-72

This commit updates URLs to prefer the https protocol. Redirects are not followed to avoid accidentally expanding intentionally shortened URLs (i.e. if using a URL shortener).

# Fixed URLs

## Fixed But Review Recommended
These URLs were fixed, but the https status was not OK. However, the https status was the same as the http request or http redirected to an https URL, so they were migrated. Your review is recommended.

* [ ] http://groovy.codehaus.org/Grape (UnknownHostException) with 1 occurrences migrated to:
  https://groovy.codehaus.org/Grape ([https](https://groovy.codehaus.org/Grape) result UnknownHostException).

## Fixed Success
These URLs were switched to an https URL with a 2xx status. While the status was successful, your review is still recommended.

* [ ] http://creativecommons.org/licenses/by-nd/3.0/ with 1 occurrences migrated to:
  https://creativecommons.org/licenses/by-nd/3.0/ ([https](https://creativecommons.org/licenses/by-nd/3.0/) result 200).
* [ ] http://docs.spring.io/spring-boot/docs/ with 6 occurrences migrated to:
  https://docs.spring.io/spring-boot/docs/ ([https](https://docs.spring.io/spring-boot/docs/) result 200).
* [ ] http://start.spring.io/ with 1 occurrences migrated to:
  https://start.spring.io/ ([https](https://start.spring.io/) result 200).

# Ignored
These URLs were intentionally ignored.

* http://localhost with 1 occurrences
* http://localhost:8080/actuator with 1 occurrences
* http://localhost:8080/actuator/health with 1 occurrences
* http://localhost:8080/actuator/info with 1 occurrences

fixes "initial" link in how_to_complete_this_guide.adoc

Resolves #55.

The link to see the spring boot starters source code in GitHub leads to the wrong page showing the 404 page. I updated the link to  go to the actual page that shows the code for spring boot starts

At the bottom of the document (above the footer), I added links to other guides that readers may find to be useful.