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

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https://hg.mozilla.org/integration/gaia-central/rev/5e5e2e3ee50f
Author: Jonathan Griffin <jgriffin@mozilla.com>
Desc: Merge pull request #16064 from jonallengriffin/gaiatest_version

Bug 968996 - Bump gaiatest to 0.21.5, r=zac

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https://hg.mozilla.org/integration/gaia-central/rev/63724faa0d3a
Author: Jonathan Griffin <jgriffin@mozilla.com>
Desc: Bug 968996 - Bump gaiatest to 0.21.5, r=zac
Bumping manifests a=b2g-bump