AttributeError exception in _example_from_array_spec() when array items references simple type defintion

[mtherieau]

in cases where an array items field references a simple type definition then _example_from_array_spec() throws an AttributeError exception like so:

https://travis-ci.org/Trax-air/swagger-parser/jobs/214411452#L756

Test case added in: #39
an example of this issue is in file: tests/array_ref_simple.yaml

fix is part of same PR here:
e52a7f8