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

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https://hg.mozilla.org/integration/gaia-central/rev/4d0df4482828
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #16129 from cctuan/968697

Bug 968697 - Add Makefile for system app

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https://hg.mozilla.org/integration/gaia-central/rev/59abd25ef8fc
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 968697 - Add Makefile for system app
Bumping manifests a=b2g-bump