Nexmo/api-specification no longer exists

[ovidiuenache]

As part of the test suite, we are using https://github.com/Nexmo/api-specification/tree/master/definitions to fetch example in the specProvider method of the OpenApiTest.php class. That repository no longer exists and the test fails.

I'll create a PR to fix this.

[ovidiuenache]

https://github.com/cebe/php-openapi/pull/202

[ovidiuenache]

cc @cebe