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

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https://hg.mozilla.org/integration/gaia-central/rev/839bc16aa4da
Author: Sergi Mansilla <sergi.mansilla@gmail.com>
Desc: Merge pull request #21831 from sergi/blank_screen

Bug 1038815 - Contacts can get into a 'frozen' state in some cases r=francisco

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https://hg.mozilla.org/integration/gaia-central/rev/71757326ac8c
Author: Sergi Mansilla <sergi.mansilla@gmail.com>
Desc: Bug 1038815 - Contacts can get into a 'frozen' state in some cases
Bumping manifests a=b2g-bump