Bumping gaia.json for 3 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/b73c46ad392c
Author: GaryChen, PYChen, Gary <mmm198219@gmail.com>
Desc: Merge pull request #21158 from mpizza/Bug_996461_remove_distribution_tablet

Bug 996461 - [Build System] Remove 'distribution_tablet' folder from gaia code base. r=yuren.

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https://hg.mozilla.org/integration/gaia-central/rev/0b40b7816347
Author: mpizza <mmm198219@gmail.com>
Desc: Bug 996461 - [Build System] Remove 'distribution_tablet' folder from gaia code base.

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https://hg.mozilla.org/integration/gaia-central/rev/5644f807a7d4
Author: Dave Hunt <dave.hunt@gmail.com>
Desc: Bug 1031297 - Bump gaiatest version to 0.25. r=zcampbell
Bumping manifests a=b2g-bump