Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/50872ab77f6b
Author: Fabrice Desré <fabrice@mozilla.com>
Desc: Bug 983760 - Dont show crash screen if tab is not currently active. r=daleharvey
Bumping manifests a=b2g-bump