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

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https://hg.mozilla.org/integration/gaia-central/rev/51e7f035fcf9
Author: Bob Silverberg <bob.silverberg@gmail.com>
Desc: Bug 1002541 - Bump gaiatest version to 0.23 and release to PyPI
Bumping manifests a=b2g-bump