Switch build.gradle to use https://repo.spring.io/plugins-release

[schafdog]

New to gradle building, so I could be doing it wrong.

But building fails unless I change to https for the above repo

[paulboocock]

Hi Dennis Schafroth (@schafdog)
Thanks for letting us know. You're doing the right thing updating this to https.
If you wish to open a PR to fix this then we'll happily accept it in our next release or we'll update this ourselves in the next release.