Fix AppEngine SDK updates via Makefile, `vagrant provision`

* Previously, `unzip` would ask you to confirm overwriting the previous
  AppEngine SDK, but `vagrant provision` doesn't allow input.  Adding
  `-o` command line switch makes `unzip` not ask permission.
Merge pull request #363 from reciprocity/feature/fix-appengine-upgrade

Fix AppEngine SDK updates via Makefile, `vagrant provision`