RFR: JDK-8041593: Update README-builds.html to refer to jdk9

[Erik Joelsson]

Here is a minor patch to README-builds.html to update references to jdk8 
to jdk9 and fix the part about boot jdk since we now require jdk8.

I realize there is probably a lot more that needs to be fixed in this 
file, but would like to leave that for another time.

Bug: https://bugs.openjdk.java.net/browse/JDK-8041593
Webrev: http://cr.openjdk.java.net/~erikj/8041593/webrev.root.01/

/Erik