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

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https://hg.mozilla.org/integration/gaia-central/rev/877751e7823f
Author: Greg Weng <snowmantw@gmail.com>
Desc: Merge pull request #16117 from snowmantw/issue968657-make-monitor

Bug 968657 - Add |make watch| to monitor files change and build for specific app

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https://hg.mozilla.org/integration/gaia-central/rev/3b6710748bc8
Author: Greg Weng <snowmantw@gmail.com>
Desc: Bug 968657 - Add |make watch| to monitor files change and build for specific app.
Bumping manifests a=b2g-bump