hbase: require Java 8 exactly

[apjanke]

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Addresses #19696 by requiring Java 8 exactly.

The install and test for hbase are not currently failing in the presence of Java 9, but when you actually use hbase on a machine with Java 9 installed, as is done in brew install -s opentsdb, then it throws Java 9 related errors.

[ilovezfs]

Nice catch! Thanks @apjanke! It would be cool if the test block could catch that so someone doesn't accidentally put this back.