SPARK-1737: Warn rather than fail when Java 7+ is used to create distributions

Also moves a few lines of code around in make-distribution.sh.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #669 from pwendell/make-distribution and squashes the following commits:

8bfac49 [Patrick Wendell] Small fix
46918ec [Patrick Wendell] SPARK-1737: Warn rather than fail when Java 7+ is used to create distributions.