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

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https://hg.mozilla.org/integration/gaia-central/rev/c8d21660db74
Author: Julien Wajsberg <felash@gmail.com>
Desc: Merge pull request #15466 from julienw/956276-duplicate-event

Bug 956276 - [ZTE]-Two or more instances of open activity will be launch...

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https://hg.mozilla.org/integration/gaia-central/rev/ac7a042f79c7
Author: Julien Wajsberg <felash@gmail.com>
Desc: Bug 956276 - [ZTE]-Two or more instances of open activity will be launched after received files via bluetooth; r=etienne
Bumping manifests a=b2g-bump