Change field naming in VulnerabilitySeverity model

scoring_system_identifier is changed to scoring_system

Signed-off-by: Shivam Sandbhor <shivam.sandbhor@gmail.com>

Author: sbs2001

vulnerabilities/fixtures/debian.json

@@ -4,26 +4,26 @@
   "pk": 1,
   "fields": {
     "cve_id": "CVE-2014-8242",
-    "summary": "",
-    "cvss": null
+    "summary": ""
+    
   }
 },
 {
   "model": "vulnerabilities.vulnerability",
   "pk": 2,
   "fields": {
     "cve_id": "CVE-2009-1382",
-    "summary": "",
-    "cvss": null
+    "summary": ""
+    
   }
 },
 {
   "model": "vulnerabilities.vulnerability",
   "pk": 3,
   "fields": {
     "cve_id": "CVE-2009-2459",
-    "summary": "",
-    "cvss": null
+    "summary": ""
+    
   }
 },
 {