Enable JDK8 PR validation to avoid breaking build for Java8

[ktoso]

Since Java 8 is more strict about JavaDocs projects which compile under 6 / 7, may not necessarily compile under J8, causing unnecessary trouble.

We should validate the build on both JDK 6 (to avoid accidental use of JDK8 features) and JDK 8 (to make sure it's compile-able under 8).

Most recently problem was bumped into during: #252 (comment)

(Aside: One small issue, not specifically related to this PR, I had to install Java 7 to build the TCK as it doesn't build using Java 8 - small JavaDoc issue. Java 7 will be EOL in a month)