How to force SBT to use Java 8?

You need the following on your build.sbt file.

javacOptions ++= Seq("-source", "1.8", "-target", "1.8", "-Xlint")

initialize := {
  val _ = initialize.value
  val javaVersion = sys.props("java.specification.version")
  if (javaVersion != "1.8")
    sys.error("Java 1.8 is required for this project. Found " + javaVersion + " instead")
}