Add lock for remark-mdx

Closes GH-325.

Author: wooorm

package.json

@@ -66,7 +66,7 @@
     "rehype-parse": "^7.0.0",
     "rehype-retext": "^2.0.1",
     "remark-frontmatter": "^2.0.0",
-    "remark-mdx": "^2.0.0-next.7",
+    "remark-mdx": "2.0.0-next.7",
     "remark-message-control": "^6.0.0",
     "remark-parse": "^8.0.0",
     "remark-retext": "^4.0.0",