SpringBoot spring-boot-image-tests fail when trying to test a new version with no published snapshots

The tests currently rely on being able to resolve the Gradle plugin and its dependencies and the web starter and its dependencies from http://repo.spring.io. This means it is currently impossible to run the system tests for a new version of Boot until a "main" build has run and published snapshots for that version. Ideally, we should be able to change the version in gradle.properties and then run systemTest and it would pass.