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

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https://hg.mozilla.org/integration/gaia-2_0/rev/7f9d1558ff1b
Author: crdlc <crdlc@tid.es>
Desc: Bug 1026257 - Would like to be able to set default number of colums in vertical homescreen at build time
Bumping manifests a=b2g-bump