sbt: 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.

http://www.scala-sbt.org/release/docs/Getting-Started/Setup.html
http://stackoverflow.com/questions/15957183/outofmemoryerror-from-sbt

Closes #28815.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>