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

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https://hg.mozilla.org/integration/gaia-central/rev/42774c3a6de3
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #15347 from crdlc/bug-951281

Bug 951281 - [Download Manager] Allow downloaded files to be shared

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https://hg.mozilla.org/integration/gaia-central/rev/18a2a8dd319e
Author: crdlc <crdlc@tid.es>
Desc: Bug 951281 - [Download Manager] Allow downloaded files to be shared
Bumping manifests a=b2g-bump