Bumping gaia.json for 1 gaia-1_2 revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-1_2/rev/bc15df8b3e1a
Author: Dave Hunt <dave.hunt@gmail.com>
Desc: Bug 915572 - Allow data/gaia/sdcard cleanup to be overridden by tests