Modify source tree filter for JDK8 compatibility.

The simpler tag filter on `com.sun.tools.javac.tree.JCTree` is
complicated by this changing from a integer enumeration in JDK7 to
a actual enum in JDK8. Use `com.sun.source.tree.Tree$Kind` instead,
and look at its *internal interface* (filter for
`com.sun.source.tree/ClassTree`), since ENUM and INTERFACE `Kind`s are
defined independently of CLASS in JDK7+, but not JDK6.
Add JDK8 to CI builds.
Merge pull request #72 from jeffvalk/master

Modify source tree filter for JDK8 compatibility.