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

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https://hg.mozilla.org/integration/gaia-central/rev/f6591d2cdd8d
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #12961 from gasolin/issue-905501

Bug 905501 - [FlatFish] Write guideline for multiple resolution device s..., r=timdream

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https://hg.mozilla.org/integration/gaia-central/rev/2e5857a0496d
Author: gasolin <gasolin@gmail.com>
Desc: Bug 905501 - [FlatFish] Provide README for tablet customization

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https://hg.mozilla.org/integration/gaia-central/rev/a7527ea8c8b2
Author: James Lal <james@lightsofapollo.com>
Desc: Merge pull request #13220 from lightsofapollo/revert-9ab17bf1759b8abd5e1de7e71c98f617a87a7a25

Revert "Merge pull request #13161 from     luke-chang/919370_unit_test_for_pinyin"

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https://hg.mozilla.org/integration/gaia-central/rev/d8f9dd372370
Author: James Lal <james@lightsofapollo.com>
Desc: Revert "Merge pull request #13161 from luke-chang/919370_unit_test_for_pinyin"

This reverts commit 9ab17bf1759b8abd5e1de7e71c98f617a87a7a25, reversing
changes made to 1dd8c7d1209cdf23bd8bdfd1878fcdcdc58fb2bc.