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

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https://hg.mozilla.org/integration/gaia-central/rev/6621a724866b
Author: Julien Wajsberg <felash@gmail.com>
Desc: Merge pull request #20157 from julienw/1021844-sms-firefox

Bug 1021844 - [Messages] Can't load the application in Firefox r=schung

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https://hg.mozilla.org/integration/gaia-central/rev/5f47c4bf5c11
Author: Julien Wajsberg <felash@gmail.com>
Desc: Bug 1021844 - [Messages] Can't load the application in Firefox r=schung
Bumping manifests a=b2g-bump