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

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https://hg.mozilla.org/integration/gaia-central/rev/b8c4a5e35bff
Author: Marshall Culpepper <marshall.law@gmail.com>
Desc: Merge pull request #17673 from marshall/ftuPing

Bug 969101: Initial implementation of a first-time use ping.

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https://hg.mozilla.org/integration/gaia-central/rev/15fc67e7fb67
Author: Marshall Culpepper <marshall@mozilla.com>
Desc: Bug 969101: Initial implementation of a first-time use ping. r=fabrice

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https://hg.mozilla.org/integration/gaia-central/rev/3890ee2a25d3
Author: Carsten Book -Tomcat- <cbook@mozilla.com>
Desc: Revert "Bug 982033 - change layout manager instantiate place to solve app_window dependency" to resolve test bustage

This reverts commit 5b499bc5ffbc4fe62d90154dc0d3233bcf30ddd0.
Bumping manifests a=b2g-bump