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

========

https://hg.mozilla.org/integration/gaia-central/rev/15ffd09854fd
Author: Gareth Aye <gaye@mozilla.com>
Desc: Merge pull request #15586 from gaye/i-am-so-sorry

Revert "Merge pull request #15585 from gaye/email-mass-reenable"

========

https://hg.mozilla.org/integration/gaia-central/rev/f637aad2e45c
Author: Gareth Aye <gaye@mozilla.com>
Desc: Revert "Merge pull request #15585 from gaye/email-mass-reenable"

This reverts commit 67828ef861a77e991cb0e2739973925ac82d7658, reversing
changes made to 55b1b6d56afbfd13a3ae05e530a5511b5831ea9f.

========

https://hg.mozilla.org/integration/gaia-central/rev/337d47d0209d
Author: Gareth Aye <gaye@mozilla.com>
Desc: Merge pull request #15585 from gaye/email-mass-reenable

WIP - Mass reenable email tests

========

https://hg.mozilla.org/integration/gaia-central/rev/8ad132a1e418
Author: Gareth Aye <gaye@mozilla.com>
Desc: WIP - Mass reenable email tests
Bumping manifests a=b2g-bump