openapi: add missing run_number field to start_workflow response

Closes reanahub/reana-db#186

Author: mdonadoni

reana_commons/openapi_specifications/reana_server.json

@@ -2,7 +2,7 @@
   "info": {
     "description": "Submit workflows to be run on REANA Cloud",
     "title": "REANA Server",
-    "version": "0.9.1a3"
+    "version": "0.9.1"
   },
   "paths": {
     "/account/settings/linkedaccounts/": {},
@@ -3234,6 +3234,9 @@
                 "message": {
                   "type": "string"
                 },
+                "run_number": {
+                  "type": "string"
+                },
                 "status": {
                   "type": "string"
                 },