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

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https://hg.mozilla.org/integration/gaia-central/rev/6504d6fc5814
Author: Aleh Zasypkin <aleh.zasypkin@gmail.com>
Desc: Merge pull request #20528 from azasypkin/bug-925404-carrier-header-format

Bug 925404 - [B2G] [SMS] Always include the phone number in the SMS Thread UI, even if the carrier is known. r=schung

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https://hg.mozilla.org/integration/gaia-central/rev/c270565b05e9
Author: Aleh Zasypkin <azasypkin@mozilla.com>
Desc: Bug 925404 - [B2G] [SMS] Always include the phone number in the SMS Thread UI, even if the carrier is known. r=schung
Bumping manifests a=b2g-bump