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

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https://hg.mozilla.org/integration/gaia-central/rev/ff2728add438
Author: autolander <bug.autolander@gmail.com>
Desc: Bug 1105521 - merge pull request #26500 from KevinGrandon:bug_1105521_remove_strings_sleep_menu to mozilla-b2g:master

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https://hg.mozilla.org/integration/gaia-central/rev/f13af46b1fd6
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1105521 - Remove strings from index.html for sleep menu
Bumping manifests a=b2g-bump