Reenable LiveVersionMapTests.testRamBytesUsed on JDK9

[ywelsch]

I've disabled LiveVersionMapTests.testRamBytesUsed for JDK9 on branches master, 5.x and 5.2 for now. Needs to be re-enabled once we have a Lucene release where RamUsageTester works under JDK 9.