Bumping gaia.json for 4 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/de6792f85569
Author: Fabien Cazenave <fabien@mozilla.com>
Desc: Merge pull request #16739 from mcjimenez/bug976951_v2

Bug 976951 - Customize network indicator by SIM displayed in quick settings, r=21+kaze

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https://hg.mozilla.org/integration/gaia-central/rev/f9f402cc6a6d
Author: Carmen Jimenez Cabezas <cjc@tid.es>
Desc: Bug 976951 - Customize network indicator by SIM displayed in quick settings

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https://hg.mozilla.org/integration/gaia-central/rev/7954dcdce213
Author: Mason Chang <mason@masonchang.com>
Desc: Merge pull request #16751 from changm/bug968478

Bug 968478 - Put calendar behind scrollable background. r=millermedeiros

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https://hg.mozilla.org/integration/gaia-central/rev/fda747549208
Author: Mason Chang <mchang@mozilla.com>
Desc: Bug 968478 - Put calendar behind scrollable background
Bumping manifests a=b2g-bump