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

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https://hg.mozilla.org/integration/gaia-central/rev/76cfb7597078
Author: Dietrich Ayala <autonome@gmail.com>
Desc: Merge pull request #15822 from autonome/961946

Bug 961946 - a bunch of errors at startup in desktop Firefox

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https://hg.mozilla.org/integration/gaia-central/rev/2b9b7313403f
Author: Dietrich Ayala <autonome@gmail.com>
Desc: Bug 961946 - a bunch of errors at startup in desktop Firefox
(r=kgrandon)
Bumping manifests a=b2g-bump