AttributeError exception in _get_example_from_properties() when object has no properties field

[mtherieau]

in cases where there is no properties field (e.g. properties are specified via additionalProperties) then _get_example_from_properties() throws an AttributeError exception, like so:

https://travis-ci.org/Trax-air/swagger-parser/jobs/214411452#L643

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

c80700d#diff-0d3c37b4c72efec5a9a3a1bde3230a16

The fix is part of same PR, here:
a191fb3