SI-7497 Fix scala.util.Properties.isMac

It returned wrong values because it used the java.vendor
property instead of os.name.

See http://mail.openjdk.java.net/pipermail/macosx-port-dev/2012-November/005148.html
for the reason why we don't follow developer.apple.com/library/mac/#technotes/tn2002/tn2110.