Update sbt.rb to use recommended defaults

Default brew install of `sbt` easily runs out of memory.

The addition of the flags `-Xms512M -Xmx1536M -Xss1M -XX:+CMSClassUnloadingEnabled -XX:MaxPermSize=256M` can prevent this from happening.

Based from answers in http://stackoverflow.com/questions/15957183/outofmemoryerror-from-sbt.