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

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https://hg.mozilla.org/integration/gaia-central/rev/acb6215df962
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #19899 from mschwart/bug991949

Bug 991949 - Cell broadcast ETWS messages show undefined, r=timdream

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https://hg.mozilla.org/integration/gaia-central/rev/3c8f0d8097aa
Author: Michael Schwartz <mschwart@codeaurora.org>
Desc: Bug 991949 - Cell broadcast ETWS messages show undefined

ETWS messages without a message body use a canned body depending on the
warning type.

Change-Id: I25491fd85894d334bd6a9ab63105379de495ef2a
Bumping manifests a=b2g-bump