Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/cb957e1ec137
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Bug 1017002 - [Homescreen] Disable intermittent Marionette JS failure on Travis: 1) Homescreen navigation > Going to the homescreen and back to a warm app: AssertionError: we got 2 reflows instead of 0
Bumping manifests a=b2g-bump