Bumping gaia.json for 4 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/4c6376fe6fab
Author: Julien Wajsberg <felash@gmail.com>
Desc: Merge pull request #18073 from julienw/992891-file-too-large-string

Bug 992891 - [B2G][Messages] Warning message is missing while trying to ...

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https://hg.mozilla.org/integration/gaia-central/rev/4f379be7fc47
Author: Julien Wajsberg <julien@mozilla.com>
Desc: Bug 992891 - [B2G][Messages] Warning message is missing while trying to attach a video file too large to send via MMS r=schung

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https://hg.mozilla.org/integration/gaia-central/rev/6f98eb7a9469
Author: Gareth Aye <gaye@mozilla.com>
Desc: Merge pull request #18135 from gaye/bug-974554

Bug 974554 - CalDAV parser should accept ical inside xml comments nested in <calendar-data> elements r=millermedeiros

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https://hg.mozilla.org/integration/gaia-central/rev/4f00039f84c4
Author: gaye <gaye@mozilla.com>
Desc: Bug 974554 - CalDAV parser should accept ical inside xml comments nested  in <calendar-data> elements r=millermedeiros
Bumping manifests a=b2g-bump