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

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https://hg.mozilla.org/integration/gaia-central/rev/8720f41b0ad5
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #15949 from KevinGrandon/bug_967579_indentation

Bug 967579 - Build test code style

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https://hg.mozilla.org/integration/gaia-central/rev/72e80c60e8a3
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 967579 - Build test code style r=ochameau
Bumping manifests a=b2g-bump