fix($test-utils): test failed due to wrong babel version

Author: ulivz

packages/@vuepress/test-utils/package.json

@@ -16,8 +16,8 @@
     "generator"
   ],
   "dependencies": {
-    "@babel/core": "^7.0.0-beta.47",
-    "@babel/preset-env": "^7.0.0-beta.47",
+    "@babel/core": "^7.0.0",
+    "@babel/preset-env": "^7.0.0",
     "@vue/test-utils": "^1.0.0-beta.16",
     "@vuepress/core": "^1.0.0-alpha.23",
     "@vuepress/shared-utils": "^1.0.0-alpha.23",