Making Java 1.6 minimum version required (rebased onto dev_4_4)



This is the same as gh-449 but rebased onto dev_4_4.

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Docs PR to go with https://github.com/openmicroscopy/openmicroscopy/pull/1412 - updating system requirements to Java 1.6.

Should be back-portable to dev_4_4 for the 4.4.9 release.