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

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https://hg.mozilla.org/integration/gaia-central/rev/b20cf22507ce
Author: EragonJ <eragonj@eragonj.me>
Desc: Merge pull request #13000 from EragonJ/bug-927724

Bug 927724 - [DSDS][Gaia] Voicemail webapi will be changed for DSDS

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https://hg.mozilla.org/integration/gaia-central/rev/92118e00bafb
Author: EragonJ <eragonj@eragonj.me>
Desc: Bug 927724 - [DSDS][Gaia] Voicemail webapi will be changed for DSDS

- add feature detect for Voicemail webAPI