Merge pull request #250 from pwendell/master

README incorrectly suggests build sources spark-env.sh

This is misleading because the build doesn't source that file. IMO
it's better to force people to specify build environment variables
on the command line always, like we do in every example, so I'm
just removing this doc.
(cherry picked from commit d2efe13574090e93c600adeacc7f6356bc196e6c)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>