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

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https://hg.mozilla.org/integration/gaia-central/rev/215bb3fd100d
Author: Arthur Chen <crh0716@gmail.com>
Desc: Revert "Bug 974253 - Set preferred network type on startup based on the user preference"

This reverts commit c6931e59efef682aa42ad09cab3d843669d5f8f7.
Bumping manifests a=b2g-bump