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

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https://hg.mozilla.org/integration/gaia-central/rev/73ef21ab0b3f
Author: Vivien Nicolas <vnicolas@mozilla.com>
Desc: Bug 1063740 - Home screen is shown in landscape mode when canceling out of search app. r=kgrandon
Bumping manifests a=b2g-bump