use dylan's homebrew appengine sdk by default

Summary:
we end up telling everyone to do this anyway, so why not.

this also leaves alone existing installs and upgrades installs of the sdk
that originated with dylan's sdk and provides a bit more logging.

Test Plan: ran on bare laptop.

Reviewers: dylan

Reviewed By: dylan

Differential Revision: http://phabricator.khanacademy.org/D9042