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

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https://hg.mozilla.org/integration/gaia-central/rev/b6bfb70248a6
Author: Luke Chang <lchang@mozilla.com>
Desc: Merge pull request #15150 from luke-chang/921800_cannot_relaunch_app_under_attention_screen

Bug 921800 - [Buri][fugu][Home screen][call]Can't enter into setting again during a call if the settings was in the foreground, r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/b03bd4651b62
Author: Luke Chang <lchang@mozilla.com>
Desc: Bug 921800 - [Buri][fugu][Home screen][call]Can't enter into setting again during a call if the settings was in the foreground.