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

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https://hg.mozilla.org/integration/gaia-central/rev/c521ddfeadb0
Author: Florin Strugariu <bebef_1987@yahoo.com>
Desc: Merge pull request #15116 from viorelaioia/bug_956787

Bug 956787 - Merge the email tests where we can

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https://hg.mozilla.org/integration/gaia-central/rev/df8018404bd9
Author: Viorela Ioia <viorelaioia@gmail.com>
Desc: Bug 956787 - Merge the email tests where we can
Bumping manifests a=b2g-bump