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

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https://hg.mozilla.org/integration/gaia-central/rev/77926589c07c
Author: Gareth Aye <gaye@mozilla.com>
Desc: Merge pull request #12478 from gaye/bug-902124

Bug 902124 - Followup to fix passing NPM_REGISTRY to gaia-marionette

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https://hg.mozilla.org/integration/gaia-central/rev/63253f9fd46f
Author: Gareth Aye <gaye@mozilla.com>
Desc: Bug 902124 - Followup to fix passing NPM_REGISTRY to gaia-marionette