fixed modelgen test schema

[Code-Hex]

fixed for vektah/gqlparser#202

If we don't merge this fix, tests will fail in the future because of the PR

--- FAIL: TestModelGeneration (0.00s)
    models_test.go:22:
                Error Trace:    models_test.go:22
                Error:          Received unexpected error:
                                testdata/schema.graphql:129: Type CDImplemented must implement A because it is implemented by C
                Test:           TestModelGeneration
FAIL
FAIL    github.com/99designs/gqlgen/plugin/modelgen     0.121s

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

Coverage remained the same at 74.493% when pulling 4cabaf4 on Code-Hex:fix/model-gen-implements-test into d873ff8 on 99designs:master.