Appease validator and stringify version field

From warning via

cffconert --validate

.zenodo.json

@@ -145,5 +145,5 @@
     },
     "publication_date": "2019-05-02",
     "title": "REMIND - REgional Model of INvestments and Development",
-    "version": 4.0
+    "version": "4.0"
 }

CITATION.cff

@@ -138,7 +138,7 @@ authors:
 
 
 title: REMIND - REgional Model of INvestments and Development
-version: 4.0
+version: "4.0"
 date-released: 2019-05-02
 repository-code: https://github.com/remindmodel/remind
 keywords: