For Mac OS X, update GoogleAppEngineLauncher to version 1.8.3

Test Plan:
Ran the following commands cribbed from mac-setup.sh:
    curl -s http://googleappengine.googlecode.com/files/GoogleAppEngineLauncher-1.8.3.dmg \
        -o ~/Downloads/GoogleAppEngineLauncher-1.8.3.dmg
    hdiutil attach ~/Downloads/GoogleAppEngineLauncher-1.8.3.dmg
and saw that the mounted volume contained GoogleAppEngineLauncher.app.

Reviewers: csilvers

Reviewed By: csilvers

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D3785