Make deprecation warning less severe

Just a small change. I think it's good not to scare people who are using the old options.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #810 from pwendell/warnings and squashes the following commits:

cb8a311 [Patrick Wendell] Make deprecation warning less severe
(cherry picked from commit 442808a7482b81c8de887c901b424683da62022e)

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