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

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https://hg.mozilla.org/integration/gaia-1_3/rev/56a10ac7c5b5
Author: Bob Silverberg <bob.silverberg@gmail.com>
Desc: Merge pull request #17972 from viorelaioia/bug_991659

Bug 991659 - [v1.3] Backport Music tests to v1.3

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https://hg.mozilla.org/integration/gaia-1_3/rev/5856a223f183
Author: Viorela Ioia <viorelaioia@gmail.com>
Desc: Bug 991659 - [v1.3] Backport Music tests to v1.3
Bumping manifests a=b2g-bump