Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/6dab9e3016c7
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Merge pull request #18450 from timdream/detect-gaia-location

Bug 915484 - Figure out GAIA_INSTALL_PARENT ourselves if unset, r=ochameau

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https://hg.mozilla.org/integration/gaia-central/rev/f9329944fe8c
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Bug 915484 - Figure out GAIA_INSTALL_PARENT ourselves if unset
Bumping manifests a=b2g-bump