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

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https://hg.mozilla.org/integration/gaia-central/rev/df0b991e7e71
Author: Rex KM Lee <rexboy@mozilla.com>
Desc: Merge pull request #17700 from rexboy7/bug-971512-1.4

Bug 971512 - [System] Instantiable layout manager

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https://hg.mozilla.org/integration/gaia-central/rev/4fdd280424cc
Author: Rex Lee <rexboy@mozilla.com>
Desc: Bug 971512 - [System] Instantiable layout manager
Bumping manifests a=b2g-bump