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

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https://hg.mozilla.org/integration/gaia-central/rev/91de5ff4f474
Author: GaryChen,PYChen <mmm198219@gmail.com>
Desc: Merge pull request #17504 from mpizza/Bug_961563_build_system

Bug 961563 - [TABLET] Support specifying target device type in Gaia build script. r=yurenju.

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https://hg.mozilla.org/integration/gaia-central/rev/bfc5f924e3bd
Author: mpizza <mmm198219@gmail.com>
Desc: Bug 961563 - [TABLET] Support specifying target device type in Gaia build script.
Bumping manifests a=b2g-bump