Adds Java 8 support https://github.com/Kotlin/kotlin-spark-api/pull/189 requested by https://github.com/Kotlin/kotlin-spark-api/issues/186, https://github.com/Kotlin/kotlin-spark-api/issues/183, https://github.com/Kotlin/kotlin-spark-api/issues/190. All modules except jupyter now target Java (1.)8 and should be compatible with Java (1.)8 servers. Let us know if you have any issues with it!