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

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https://hg.mozilla.org/integration/gaia-central/rev/91e051bd5473
Author: Ryan VanderMeulen <ryanvm@gmail.com>
Desc: Revert "Bug 997521 - Add prepopulated topsites and history. r=benfrancis"

This reverts commit 4d0597fcd9f283c4e4d54be63c067f77c5f1d849.
Bumping manifests a=b2g-bump