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

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https://hg.mozilla.org/integration/gaia-central/rev/df5a6aca27f2
Author: lissyx <lissyx+github@lissyx.dyndns.org>
Desc: Merge pull request #15469 from lissyx/bug879192

Bug 879192 - Inhibit update-checker service during Gaia UI Tests r=fabrice,mdas

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https://hg.mozilla.org/integration/gaia-central/rev/215f95b85885
Author: Alexandre Lissy <lissyx+github@lissyx.dyndns.org>
Desc: Bug 879192 - Inhibit update-checker service during Gaia UI Tests
Bumping manifests a=b2g-bump