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

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https://hg.mozilla.org/integration/gaia-central/rev/914a39fc598d
Author: Fernando Jiménez Moreno <ferjmoreno@gmail.com>
Desc: Merge pull request #15683 from ferjm/bug939021.mozpay.tests

Bug 939021 - [UITest][API] Fix and improve mozPay tests. r=zac

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https://hg.mozilla.org/integration/gaia-central/rev/cfeb07a51508
Author: Fernando Jiménez <ferjmoreno@gmail.com>
Desc: Bug 939021 - [UITest][API] Fix and improve mozPay tests. r=zac
Bumping manifests a=b2g-bump