Bumping gaia.json for 3 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/576009750c3e
Author: Gareth Aye <gaye@mozilla.com>
Desc: Merge pull request #15434 from gaye/bug-960358

Bug 960358 - Enable test manifest

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https://hg.mozilla.org/integration/gaia-central/rev/13d129f2091d
Author: Gareth Aye <gaye@mozilla.com>
Desc: Bug 960358 - Enable test manifest

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https://hg.mozilla.org/integration/gaia-central/rev/bab29c50aa5f
Author: Gabriele Svelto <gsvelto@mozilla.com>
Desc: Bug 947234 - Do not lazy load sources in the startup path r=julienw
Bumping manifests a=b2g-bump