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

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https://hg.mozilla.org/integration/gaia-central/rev/c871c880ca81
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #16897 from crdlc/bug-979795

Bug 979795 - [Download Manager] Error is displayed if you try to delete ...

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https://hg.mozilla.org/integration/gaia-central/rev/6d2c6da094f6
Author: crdlc <crdlc@tid.es>
Desc: Bug 979795 - [Download Manager] Error is displayed if you try to delete an incomplete download without SD card
Bumping manifests a=b2g-bump