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

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https://hg.mozilla.org/integration/gaia-central/rev/4c5a91931b02
Author: Albert <acperez@tid.es>
Desc: Merge pull request #13485 from acperez/bug-929602

Bug 929602 - Disable setting of origin in metadata.json in external apps

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https://hg.mozilla.org/integration/gaia-central/rev/af7ef73156ab
Author: Albert <acperez@tid.es>
Desc: Bug 929602 - Disable setting of origin in metadata.json in external apps