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

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https://hg.mozilla.org/integration/gaia-central/rev/503e799ca31f
Author: James Burke <jrburke@gmail.com>
Desc: Merge pull request #21303 from awiss/signatures

Bug 847168 - [User Story] Email Signature r=jrburke

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https://hg.mozilla.org/integration/gaia-central/rev/3c8dba9ceb98
Author: Alex Wissmann <awiss@mozilla.com>
Desc: Bug 847168 - [User Story] Email Signature
Bumping manifests a=b2g-bump