Hardcoded checked version to v4 in gscan

refs https://github.com/TryGhost/Toolbox/issues/240

- As gscan's canary is about to be aliased to upcoming v5 version current version of Ghost should be using an explicit v4 checks.
- These will change to v5 once Ghost is doing final preparations for v5 release (on the branch most likely)

core/server/services/themes/validate.js

@@ -28,20 +28,20 @@ const check = async function check(theme, isZip) {
         debug('zip mode');
         checkedTheme = await gscan.checkZip(theme, {
             keepExtractedDir: true,
-            checkVersion: 'canary',
+            checkVersion: 'v4',
             labs: labs.getAll()
         });
     } else {
         debug('non-zip mode');
         checkedTheme = await gscan.check(theme.path, {
-            checkVersion: 'canary',
+            checkVersion: 'v4',
             labs: labs.getAll()
         });
     }
 
     checkedTheme = gscan.format(checkedTheme, {
         onlyFatalErrors: config.get('env') === 'production',
-        checkVersion: 'canary'
+        checkVersion: 'v4'
     });
 
     debug('End: Check');