sbt の %% と % の違い
groupID %% artifactID % revision と groupID % artifactID % revision の違い。
If you use groupID %% artifactID % revision instead of groupID % artifactID % revision (the difference is the double %% after the groupID), SBT will add your project’s Scala version to the artifact name.
https://stackoverflow.com/questions/17461453/build-scala-and-symbols-meaning