Enable Java 8 build in travis (in addition to Java 7)

Java 8 is out, so it'd be good to start working towards supporting it properly. Here's a travis and maven change to enable builds on a JDK8 (not taking advantage of any Java8 features). 

There are 2 incompatibilities which I've resolved using profiles:
* findbugs doesn't yet support Java 8, so I've introduced a profile which disables it under Java8.
* JDK8 introduces a new strict mode to javadoc which makes Java7 compatible javadoc break the build, so I've introduced a profile which disables it under Java8.