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

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https://hg.mozilla.org/integration/gaia-central/rev/1be997d457e2
Author: William Lachance <wrlach@gmail.com>
Desc: Merge pull request #16413 from wlach/973924

Bug 973924 - Bump gaiatest to 0.21.7, r=zac

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https://hg.mozilla.org/integration/gaia-central/rev/91a74193e426
Author: William Lachance <wlachance@mozilla.com>
Desc: Bug 973924 - Bump gaiatest to 0.21.7, r=zac
Bumping manifests a=b2g-bump