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

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https://hg.mozilla.org/integration/gaia-central/rev/c521cfcc0db7
Author: RickyChien <ricky060709@gmail.com>
Desc: Merge pull request #16207 from RickyChien/bug-971647

Bug 971647 - Ignore load script error in blanket r=julienw

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https://hg.mozilla.org/integration/gaia-central/rev/a9782c719629
Author: Ricky Chien <ricky060709@gmail.com>
Desc: Bug 971647 - Ignore load script error in blanket
Bumping manifests a=b2g-bump