Travis-CI: Enable JDK8
java 8 doclint: allow <p/> in javadocs by disabling strict HTML4.0 compliance

* by default java 8 doclint consider <p/> tags as errors
  https://bugs.openjdk.java.net/browse/JDK-8020619
java 8: disable findbugs as findbugs-maven-plugin doesn't yet support jdk8

* current findbugs-maven-plugin 2.5.4 is based on findbugs 2.0.3,
  while JDK8 is supported starting findbugs 2.1.0
java 8 doclint: resolve errors and warnings for missed table caption, @return and @param