sbt: update to 0.13.15
sbt: update to 0.13.15

holdenk/spark-testing-base,holdenk/spark-testing-base,snithish/spark-testing-base,snithish/spark-testing-base,snithish/spark-testing-base,snithish/spark-testing-base,holdenk/spark-testing-base,holdenk/spark-testing-base