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

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https://hg.mozilla.org/integration/gaia-central/rev/4a8b758a638c
Author: Gregor Wagner <anygregor@gmail.com>
Desc: Merge pull request #13265 from mcjimenez/bug930544

Bug 930544 - Allow installing applications on several empty screens

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https://hg.mozilla.org/integration/gaia-central/rev/74e0bbc3791a
Author: Carmen Jimenez Cabezas <cjc@tid.es>
Desc: Bug 930544 - Allow installing applications on several empty screens