Updated default Ghost compatibility in release script

no issue

- not important, but it saves manually entering it during the release
  process
- Casper is only compatible with Ghost 4.0 moving forwards

gulpfile.js

@@ -121,7 +121,7 @@ exports.release = async () => {
             type: 'input',
             name: 'compatibleWithGhost',
             message: 'Which version of Ghost is it compatible with?',
-            default: '3.0.0'
+            default: '4.0.0'
         }]);
 
         const compatibleWithGhost = result.compatibleWithGhost;