Disable doclint for all groups but accessibility and reference.

With JDK 8, doclint has been added to Javadoc. It is enabled by default
but it is too strict for JUnit. The only groups that did not report
issues are accessibility and reference.

Additionally doclint is disabled for JDK < 8, because Javadoc does not
accept the -Xdoclint argument in JDKs before JDK 8.
Fix Javadoc issues reported by JDK8's doclint.
Create javadoc on Travis CI.

The build should fail for Javadoc issues.