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

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https://hg.mozilla.org/integration/gaia-central/rev/591b714a4728
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #15804 from crdlc/bug-964724

Bug 964724 - Hide regular Offline Error Screen When Connectivity Resumes...

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https://hg.mozilla.org/integration/gaia-central/rev/815d055bbcc1
Author: crdlc <crdlc@tid.es>
Desc: Bug 964724 - Hide regular Offline Error Screen When Connectivity Resumes - App Foreground and Background
Bumping manifests a=b2g-bump