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/a15fc016db8f
Author: Sebastiaan de Haan <me@sebastiaandehaan.nl>
Desc: Bug 935964 - Bump gaiatest to 0.19. r=dhunt