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

========

https://hg.mozilla.org/integration/gaia-central/rev/7f3fb38df1ee
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #15265 from ferjm/bug959227.fxa.uitests.revert

Bug 959227 - Revert UITest changes introduced in bug 929388. r=zac

========

https://hg.mozilla.org/integration/gaia-central/rev/fb6b76ab28ed
Author: Fernando Jiménez <ferjmoreno@gmail.com>
Desc: Bug 959227 - Revert UITest changes introduced in bug 929388. r=zac