Switched gscan to validate against v5 set of rules

refs https://github.com/TryGhost/Toolbox/issues/226

- We are on the finish line to release 5.0 - the instance should be validating it's themes agains v5 set of rules.
- The prominent deprecations are `{{author}}` and `@blog` helpers removals - will throw 'fatal' errors when encountered.

core/server/services/themes/validate.js

@@ -22,7 +22,7 @@ const check = async function check(theme, isZip) {
     debug('Begin: Check');
     // gscan can slow down boot time if we require on boot, for now nest the require.
     const gscan = require('gscan');
-    const checkedVersion = 'v4';
+    const checkedVersion = 'v5';
     let checkedTheme;
 
     if (isZip) {

test/utils/fixtures/themes/test-theme/package.json

@@ -3,9 +3,6 @@
     "description": "The default personal blogging theme for Ghost. Beautiful, minimal and responsive.",
     "demo": "https://demo.ghost.io",
     "version": "2.4.2",
-    "engines": {
-        "ghost-api": "v4"
-    },
     "license": "MIT",
     "screenshots": {
         "desktop": "assets/screenshot-desktop.jpg",