JDK version in docs is not updated

Java 8 is required to building from source.

In 'install' page, it is 1.8 (151+)
http://zeppelin.apache.org/docs/0.9.0-preview2/quickstart/install.html

but in 'build' page, it's 1.7
http://zeppelin.apache.org/docs/0.9.0-preview2/setup/basics/how_to_build.html#how-to-build-zeppelin-from-source