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

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https://hg.mozilla.org/integration/gaia-central/rev/fcb4b4cde828
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #13729 from MBRSL/bug-938966-wifi

Bug 938966 - [UITest][HW test] WiFi test, r=gasolin

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https://hg.mozilla.org/integration/gaia-central/rev/75d517c4aa85
Author: Tom Jao <tjao@mozilla.com>
Desc: Bug 938966 - [UITest][HW test] WiFi test