Fix for working with latest version

It seems that the latest version of pyMISP or MISP itself has changed it data structure. After reviewing the response, this change seemed logical and it is working (at least for me).

misp_taxii_hooks/hooks.py

@@ -66,7 +66,7 @@ def post_stix(manager, content_block, collection_ids, service_id):
     for attrib in values:
         log.info("Checking for existence of %s", attrib)
         search = MISP.search("attributes", values=str(attrib))
-        if search["response"] != []:
+        if search["response"]["Attribute"] != []:
             # This means we have it!
             log.info("%s is a duplicate, we'll ignore it.", attrib)
             package.attributes.pop([x.value for x in package.attributes].index(attrib))