Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/6739781fb8d0
Author: Gareth Aye <gaye@mozilla.com>
Desc: Merge pull request #21716 from gaye/store-promises

Bug 1027725 - Make calendar app async operations use promises

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https://hg.mozilla.org/integration/gaia-central/rev/e3bf4910ad88
Author: gaye <gaye@mozilla.com>
Desc: Bug 1027725 - Make calendar app async operations use promises
Bumping manifests a=b2g-bump