Definition references don't work without x-oad-type attributes

[mgeorge]

If you load a spec that does not include x-oad-type attributes then references to definitions do not work.

Relates to #350.

Steps to reproduce:

• Create a response definition.
• Link it to a path response as a reference.
• Export as YAML
• Remove the x-oad-type references which cause problems in other editors from the YAML.
• Reimport the modified YAML.
• View the 'Rich Preview'