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

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https://hg.mozilla.org/integration/gaia-central/rev/18d39cd2056f
Author: Marcus Cavanaugh <m@mcav.com>
Desc: Merge pull request #17827 from mcav/email-onnext

Bug 990171 - [Email] Fix NPE in FormNavigation card handling. r=jrburke

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https://hg.mozilla.org/integration/gaia-central/rev/f9378191237a
Author: Marcus Cavanaugh <m@mcav.com>
Desc: Bug 990171 - [Email] Fix NPE in FormNavigation card handling.
Bumping manifests a=b2g-bump