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

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https://hg.mozilla.org/integration/gaia-central/rev/875a61f24d09
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #16070 from AndreiH/bug968854

Bug 968854 - test_ftu_skip_tour.py fails because bug 965912

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https://hg.mozilla.org/integration/gaia-central/rev/61f2ab1b81e6
Author: Andrei Hutusoru <andreihutusoru@P5069.(none)>
Desc: Bug 968854 - test_ftu_skip_tour.py fails because bug 965912

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https://hg.mozilla.org/integration/gaia-central/rev/e0d979236333
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #16016 from gentlefolk/964252

Bug 964252 - Move FakeUpdateChecker code into a method or class

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https://hg.mozilla.org/integration/gaia-central/rev/41cc437a4f21
Author: gentlefolk <cemacken@gmail.com>
Desc: Bug 964252 - Move FakeUpdateChecker code into a method or class r=zcampbell
Bumping manifests a=b2g-bump