Bumping gaia.json for 1 gaia-1_2 revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-1_2/rev/e040f250fb37
Author: Ryan VanderMeulen <rvandermeulen@mozilla.com>
Desc: Merge pull request #12226 from darchons/bug916205-create-and-enable-dynamic-ua

Remove UA override list from Gaia(cherry picked from commit 18bd102d833e858e97d60db5c05607e172a10859)