Added Content-Version header

refs https://github.com/TryGhost/Toolbox/issues/222

- Without Admin API versioning through URL, Content-Versioning header will be the best way to suggest an expected version of the resource (formatting). Having this header in place should minimize the ammount of breaking changes and would allow for the server to respond accordingly for a particular client version

ghost/admin/app/services/ajax.js

@@ -175,6 +175,7 @@ class ajaxService extends AjaxService {
     get headers() {
         return {
             'X-Ghost-Version': config.APP.version,
+            'Content-Version': 'v4',
             'App-Pragma': 'no-cache'
         };
     }