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

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https://hg.mozilla.org/integration/gaia-central/rev/888d0e718e08
Author: Jan Jongboom <janjongboom@gmail.com>
Desc: Merge pull request #15835 from comoyo/build-keyboard

Bug 964216 - Use build_stage to build keyboard app. r=janjongboom

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https://hg.mozilla.org/integration/gaia-central/rev/b77ac420c125
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 964216 - Use build_stage to build keyboard app
Bumping manifests a=b2g-bump