Bumping gaia.json for 4 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/6c4c3328e7d4
Author: Kyle Machulis <kyle@nonpolynomial.com>
Desc: Merge pull request #23294 from qdot/revert-revert-938171

Revert revert 938171

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https://hg.mozilla.org/integration/gaia-central/rev/e3a01fb72b2e
Author: Kyle Machulis <kyle@nonpolynomial.com>
Desc: Bug 938171 - Add MockVersionHelper to bootstrap test for app_migrator module

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https://hg.mozilla.org/integration/gaia-central/rev/184a276517d0
Author: Kyle Machulis <kyle@nonpolynomial.com>
Desc: Revert "Revert "Bug 938171 - Add app_migrator module include to bootstrap unit test""

This reverts commit 0acb88a5663af1fa4ec8beb84b7cb657acba19b2.

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https://hg.mozilla.org/integration/gaia-central/rev/2b8df8b14335
Author: Kyle Machulis <kyle@nonpolynomial.com>
Desc: Revert "Revert "Bug 938171 - Browser Bookmark Migration to Homescreen in Gaia""

This reverts commit d35f1c9a22c87e96f0289135955f7316900de6ab.
Bumping manifests a=b2g-bump