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

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https://hg.mozilla.org/integration/gaia-central/rev/28bd755bb911
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #19958 from mozilla-milan/master

Bug 1018233: Missing list item closing tag.  r=vingtetun

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https://hg.mozilla.org/integration/gaia-central/rev/7b835e02693b
Author: Milan Sreckovic <milan@mozilla.com>
Desc: Bug 1018233: Missing list item closing tag.  r=vingtetun
Bumping manifests a=b2g-bump