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

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https://hg.mozilla.org/integration/gaia-central/rev/2783ae9769c1
Author: Borja Salguero <borjasalguero@users.noreply.github.com>
Desc: Merge pull request #16663 from borjasalguero/ftu_tutorial

Bug 976535 - FTE Tutorial refactor r=fcampo, ejchen

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https://hg.mozilla.org/integration/gaia-central/rev/70a95f1ea1fb
Author: borjasalguero <fbsc@tid.es>
Desc: Bug 976535 - [FTE] Refactor tutorial r=fcampo, ejchen
Bumping manifests a=b2g-bump