Remove requirement for JDK5 when building docs

Along with everyone else (https://issues.apache.org/jira/browse/HADOOP-7072), we can remove the archaic requirement for JDK5 to build the docs. This patch does so. Verified docs are built correctly. My enmity for forrest remains (although 0.9 was just released), and Howl may be a good opportunity to avoid repeating the sins of the past and introduce a new system, such as Jekyll.