Bumping gaia.json for 3 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/a0c2339eb78b
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #15979 from pivanov/bug-965719

Bug 965719 - [Browser] Add to homescreen background needs to be updated

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https://hg.mozilla.org/integration/gaia-central/rev/078746457868
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 965719 - [Browser] Add to homescreen background needs to be updated

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https://hg.mozilla.org/integration/gaia-central/rev/e7cf9e6c3976
Author: Zac <zcampbell@mozilla.com>
Desc: Bug 968346 - Investigate test_persona_cookie.py a=testonly r=me
Bumping manifests a=b2g-bump