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

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https://hg.mozilla.org/integration/gaia-central/rev/b7aaa057a974
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #15832 from zacc/bug959217

Bug 959217 - Switch tests to the new Marionette Wait class

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https://hg.mozilla.org/integration/gaia-central/rev/a68b1549db02
Author: Zac Campbell <zcampbell@mozilla.com>
Desc: Bug 959217 - Switch tests to the new Marionette Wait class (refine dhunt's pull)

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https://hg.mozilla.org/integration/gaia-central/rev/2e27bb5f7d76
Author: Dave Hunt <dave.hunt@gmail.com>
Desc: Bug 959217 - Switch tests to the new Marionette Wait class. r=bsilverberg r=zcampbell
Bumping manifests a=b2g-bump