Bumping gaia.json for 4 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/4b20ba0aa48d
Author: Rex KM Lee <rexboy@mozilla.com>
Desc: Merge pull request #13213 from rexboy7/fix-bug912005

Bug 912005 - [Dialer][CDMA] Inform bluetooth of call manipulations, r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/ac9667038774
Author: Rex Lee <rexboy@mozilla.com>
Desc: Bug 912005 - [Dialer][CDMA] Inform bluetooth of call manipulations

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https://hg.mozilla.org/integration/gaia-central/rev/823f786bbd8a
Author: Arthur Chen <crh0716@gmail.com>
Desc: Merge pull request #13216 from soumyakanti/Bug_931379

Bug 931379 - Remove Pinyin (Simplified Chinese) selection from Developer... r=crh0716

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https://hg.mozilla.org/integration/gaia-central/rev/acb0747cc887
Author: Soumya Kanti Chakraborty <soumyakanti.chakraborty@gmail.com>
Desc: Bug 931379 - Remove Pinyin (Simplified Chinese) selection from Developer>Keyboard layouts