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

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https://hg.mozilla.org/integration/gaia-central/rev/74877b52dcb3
Author: Fabrice Desré <fabrice@desre.org>
Desc: Merge pull request #11228 from karlcow/master

Bug 899162 - Remove UA override for tumblr.com and arstechnica.com r=lmandel

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https://hg.mozilla.org/integration/gaia-central/rev/2b169af1a2a1
Author: karl <karl+github@la-grange.net>
Desc: Remove UA override for tumblr.com (Bug 826361) and arstechnica.com (Bug 843149)