Use MaxMetaspaceSize rather than MaxPermSize on Java 8

@jroper points out that 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