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

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https://hg.mozilla.org/integration/gaia-central/rev/a4df4b081ede
Author: Germán Toro del Valle <gtorodelvalle@gmail.com>
Desc: Merge pull request #15727 from gtorodelvalle/contacts-bug-962989-message-to-Facebook-contact

Bug 962989 - [messages] sending message to a Facebook contact doesn't work (r=jmcanterafonseca)

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https://hg.mozilla.org/integration/gaia-central/rev/0b87cc86a46a
Author: Germán Toro del Valle <gtorodelvalle@gmail.com>
Desc: Bug 962989 - [messages] sending message to a Facebook contact doesn't work
Bumping manifests a=b2g-bump