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

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https://hg.mozilla.org/integration/gaia-central/rev/b6840600c75d
Author: Wilson Page <wilsonpage@me.com>
Desc: Merge pull request #23320 from wilsonpage/1015292

Bug 1015292 - [Calendar] Update to use gaia-header

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https://hg.mozilla.org/integration/gaia-central/rev/122d52faede6
Author: Yan Or <yor@mozilla.com>
Desc: Bug 1015292 - [Calendar] Update to use gaia-header
Bumping manifests a=b2g-bump