Merge pull request #444 from mateiz/py-version

Clarify that Python 2.7 is only needed for MLlib
(cherry picked from commit 4f0c361b0e140f5f6879f019b2e1a16c683c705c)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
Change log for release 0.9.0-incubating
[maven-release-plugin] prepare release v0.9.0-incubating
[maven-release-plugin] prepare for next development iteration