Previously, if the main project hadn't been installed in the local Maven repository (./gradlew install), rest-notes-spring-data-rest would fail to build as the plugin dependency, spring-restdocs-asciidoctor, was not available. This commit corrects the problem by adding repo.spring.io/libs-snapshot as a plugin repository in addition to its existing declaration as a "normal" repository. Fixes gh-382 Closes gh-383