BUILD: Redirect stderr to stdout for `java -help` in setup.py

in Ubuntu 14.04 on x86_64, running java version "1.7.0_65"
OpenJDK Runtime Environment (IcedTea 2.5.3) (7u71-2.5.3-0ubuntu0.14.04.1)
OpenJDK 64-Bit Server VM (build 24.65-b04, mixed mode)
the output of -help is to stderr, which creates a lot of noise during
the probe for java in setup.py

We avoid this noise by redirection of stderr to stdout, which is returned
by the subprocess.check_output() call.