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

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https://hg.mozilla.org/integration/gaia-central/rev/15d773c6e01f
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #13083 from timdream/keyboard-mozapptype

Bug 928270 - Set keyboard iframe attribute mozapptype=inputmethod, r=gchen

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https://hg.mozilla.org/integration/gaia-central/rev/6a863bd14d63
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 928270 - Set keyboard iframe attribute mozapptype=inputmethod