Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/e28e28e2a55a
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 966882 - Enable test_privileged_app_contacts_prompy.py a=testonly r=me
Bumping manifests a=b2g-bump