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

========

https://hg.mozilla.org/integration/gaia-central/rev/6a6a9878f8a9
Author: Andrew Sutherland <asutherland@asutherland.org>
Desc: Merge pull request #23058 from asutherland/email-unread-msgs-backend

Bug 800402 - [email/backend] UI: Show the number of unread messages in the message-list header after the folder name. r=asuth

========

https://hg.mozilla.org/integration/gaia-central/rev/310a982044cb
Author: Alex Wissmann <awiss@wharton.upenn.edu>
Desc: Bug 800402 - [email/backend] UI: Show the number of unread messages in the message-list header after the folder name. r=asuth

Land the back-end parts of bug 800402 and bug 888955 which it depends on.
The UI parts are separable and we're landing the back-end stuff on its own to
avoid back-end bit-rot.

The changes in here exactly correspond to those in:
https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/328
Bumping manifests a=b2g-bump