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

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https://hg.mozilla.org/integration/gaia-central/rev/3b850fa17c88
Author: Germán Toro del Valle <gtorodelvalle@gmail.com>
Desc: Merge pull request #15107 from gtorodelvalle/contacts-bug-957577-multiple-navigationStack-instances

Bug 957577 - [Contacts] navigation.js should support several concurrent instances of navigationStack

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https://hg.mozilla.org/integration/gaia-central/rev/f77e2593d5dd
Author: Germán Toro del Valle <gtorodelvalle@gmail.com>
Desc: Bug 957577 - [Contacts] navigation.js should support several concurrent instances of navigationStack.