CRM-17430 domain_version not required anymore.

CRM-17430 domain_version not required anymore.

The failing unit tests had to do with api calls without a
domain_version param, while domain_version was marked as required.

But I don't really see the point of that. Besides, before I fixed this
issue, you could call the Domain.create API without domain_version. It
would then just (incorrectly) use 3 as domain_version.

So now domain_version is not required any more, I guess this will
fix some test errors.

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* CRM-17430: Altering the domain using the API changes the CiviCRM version to 3
  https://issues.civicrm.org/jira/browse/CRM-17430