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

========

https://hg.mozilla.org/integration/gaia-central/rev/79fdd326f7c0
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Merge pull request #13981 from pivanov/bug-939209

Bug 939209 - [Action Icons - Email] Remove shadows from action icons

========

https://hg.mozilla.org/integration/gaia-central/rev/b978e893868a
Author: Pavel Ivanov <pivanov@mozilla.com>
Desc: Bug 939209 - [Action Icons - Email] Remove shadows from action icons