Bumping gaia.json for 1 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/e4da7eb21f88
Author: Gabriele Svelto <gabriele.svelto@gmail.com>
Desc: Bug 915081 - Hide the end-and-answer button during CDMA call waiting r=etienne

- Removed the end-and-answer button when in CDMA call waiting mode as it
  does not support ending the first call and answering the other one,
  the hold-and-answer button is enlarged to fill up the entire container
  under these conditions
- Added unit-tests covering the changes