Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-1_3/rev/21b21be491b2
Author: Ryan VanderMeulen <rvandermeulen@mozilla.com>
Desc: Merge pull request #16430 from theres-waldo/master

Bug 964517 - Add solid background to apps where appropriate. r=alive
Bumping manifests a=b2g-bump