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

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https://hg.mozilla.org/integration/gaia-central/rev/407020347f28
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #20637 from KevinGrandon/bug_1026759_gaia_grid_dereference

Bug 1026759 - [GaiaGrid] Re-using grid item objects causes them to not render

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https://hg.mozilla.org/integration/gaia-central/rev/d681213ecb3c
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1026759 - [GaiaGrid] Re-using grid item objects causes them to not render r=amirn
Bumping manifests a=b2g-bump