Use MaxMetaspaceSize rather than MaxPermSize on Java 8

Thanks to @jroper for the heads up: class metadata has to go _somewhere_... and it's new name is 'Metaspace' - not 'PermGen': https://github.com/sbt/sbt-launcher-package/pull/66#issuecomment-57407643

http://java.dzone.com/articles/java-8-permgen-metaspace