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

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https://hg.mozilla.org/integration/gaia-central/rev/74c5dfc0488f
Author: Gareth Aye <gaye@mozilla.com>
Desc: Merge pull request #14921 from gaye/bug-952760

Bug 952760 - Email next/previous marionette setup orange

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https://hg.mozilla.org/integration/gaia-central/rev/e584fbe6f0a8
Author: Gareth Aye <gaye@mozilla.com>
Desc: Bug 952760 - Email next/previous marionette setup orange