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

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https://hg.mozilla.org/integration/gaia-central/rev/f4370aed91df
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #17593 from bobsilverberg/plivo_call

Bug 924833 - Re-enable test_receive_call.py

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https://hg.mozilla.org/integration/gaia-central/rev/8732231c56a3
Author: Bob Silverberg <bob.silverberg@gmail.com>
Desc: Bug 924833 - Re-enable test_receive_call.py

Add a step to check that there are credits available for making a call
Add test to manifest

Address code review comments:
- Fail when Plivo balance is insufficient
- Break out arguments to PlivoUtil __init__
- Use marionette.Wait for waiting
- Add a custom PlivoError exception
- General code cleanup

Address pull request comments:
- move check for balance into make_call in the utility class
- address syntax nits
- add custom message to custom exception
- change syntax for custom exceptions
Bumping manifests a=b2g-bump