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

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https://hg.mozilla.org/integration/gaia-1_3/rev/502a27d44608
Author: Gabriele Svelto <gsvelto@mozilla.com>
Desc: Bug 969413 - Make the edit mode bottom margin rule apply only to the relevant elements r=julienw

(cherry picked from commit a0d1ec4a065613b4d8f10e3d07a0163631e0ebe8)
Bumping manifests a=b2g-bump