Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/891d459c0d6b
Author: Sam Joch <samuel@guiora.com>
Desc: Merge pull request #16034 from samjoch/bug-968780-unit-tests-in-b2g-on-mac

Bug 968780 - unit tests in b2g before script failed on mac r=julienw

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https://hg.mozilla.org/integration/gaia-central/rev/6ba2330cf088
Author: Sam Joch <samuel@guiora.com>
Desc: Bug 968780 - unit tests in b2g before script failed on mac r=julienw
Bumping manifests a=b2g-bump