fix resolving directory of relative swagger references

lib/specResolver.js

@@ -167,11 +167,12 @@ class SpecResolver {
     let slicedRefName = refName.slice(1);
     let reference = node['$ref'];
     let parsedReference = utils.parseReferenceInSwagger(reference);
+    let docDir = path.dirname(docPath);
     
     if (parsedReference.filePath) {
       //assuming that everything in the spec is relative to it, let us join the spec directory 
       //and the file path in reference.
-      docPath = utils.joinPath(self.specDir, parsedReference.filePath);
+      docPath = utils.joinPath(docDir, parsedReference.filePath);
     }
 
     return utils.parseJson(docPath).then(function (result) {