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

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https://hg.mozilla.org/integration/gaia-central/rev/c6c75a2e1654
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Revert "Bug 921424 - Replace wait for landing page after unlocking with wait for displayed app to be homescreen"

This reverts commit 381faaba87ebc6c743b3a3ef46d8b5d3b27e05ec.