Fix build to work on Java 6 on Mac OS X
Fix build to work on Java 6 on Mac OS X

Apple's Java 6 JRE doesn't have rt.jar

gzm0/scala-js,xuwei-k/scala-js,SebsLittleHelpers/scala-js,xuwei-k/scala-js,SebsLittleHelpers/scala-js,sjrd/scala-js,sjrd/scala-js,scala-js/scala-js,gzm0/scala-js,xuwei-k/scala-js,lrytz/scala-js,nicolasstucki/scala-js,sjrd/scala-js,nicolasstucki/scala-js,scala-js/scala-js,lrytz/scala-js,scala-js/scala-js