Fixed regression test

no issue

- html to plaintext was broken with c219446f85 . Ghost's API supports html intpu format only for version above v8

core/test/regression/api/v2/admin/posts_spec.js

@@ -173,6 +173,11 @@ describe('Posts API', function () {
         });
 
         it('html to plaintext', function () {
+            // NOTE: only supported in node v8 and higher
+            if (process.version.startsWith('v6.')) {
+                this.skip();
+            }
+
             return request
                 .get(localUtils.API.getApiQuery(`posts/${testUtils.DataGenerator.Content.posts[0].id}/`))
                 .set('Origin', config.get('url'))