Avoid error when rendering version diff

can't always assume relationship entities are versioned

tailbone/diffs.py

@@ -195,7 +195,7 @@ class VersionDiff(Diff):
 
                 ref = getattr(version, prop.key)
                 if ref:
-                    ref = ref.version_parent
+                    ref = getattr(ref, 'version_parent', None)
                     if ref:
                         return HTML.tag('span', c=[
                             text,