travis.yaml: bump google_appengine zipfile to 1.8.9

This pull request fixes the Python 2.7 Travis build of stashboard by updating the GAE SDK zipfile to its current version, 1.8.9. The previous version now 404's...alas.

You can see a verified successful build for Python 2.7 at https://travis-ci.org/hblanks/stashboard/jobs/18616608