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

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https://hg.mozilla.org/integration/gaia-central/rev/c20d141e5483
Author: AndreiH <andrei.hutusoru@softvision.ro>
Desc: Merge pull request #18361 from zacc/bug_996559

Bug 996559 - Xfail test_gallery_flick.py due to Marionette actions faili...

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https://hg.mozilla.org/integration/gaia-central/rev/027910aff24c
Author: Zac <zcampbell@mozilla.com>
Desc: Bug 996559 - Xfail test_gallery_flick.py due to Marionette actions failing
Bumping manifests a=b2g-bump