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

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https://hg.mozilla.org/integration/gaia-central/rev/6b4d5f000dde
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #15458 from cctuan/960114

Bug 960114 - Multiple CSS warnings shown when launching the Gallery app

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https://hg.mozilla.org/integration/gaia-central/rev/1d69a5fa7321
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 960114 - Multiple CSS warnings shown when launching the Gallery app
Bumping manifests a=b2g-bump