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

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https://hg.mozilla.org/integration/gaia-central/rev/15e078a31a90
Author: Rudy Lu <left.lu@gmail.com>
Desc: Merge pull request #12058 from luke-chang/908592_pinyin_symbol_panel

Bug 908592 - [Keyboard] Improve symbol panel layout of Pinyin IME. r=rudylu

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https://hg.mozilla.org/integration/gaia-central/rev/90dabe7f6eca
Author: Luke Chang <lchang@mozilla.com>
Desc: Bug 908592 - [Keyboard] Improve symbol panel layout of Pinyin IME