Enanle Travis-CI JDK8

Support building with JDK8.

* enable Travis OracleJDK8
* comply with more strict java 8 javadoc doclint
* disable findbugs-maven-plugin in case of JDK8, which is not yet supported at 2.5.4
* configure java 8 doclint to ignore javadoc `self-closing element not allowed` error

The last item can be resolved just by replacing javadocs `<p/>` to `<p>`, but may conflict with the exising patches. Do you think it's ok to also apply it within this PR?