Update gray-matter to v3.

Author: mgroenhoff

package.json

@@ -49,7 +49,7 @@
   "dependencies": {
     "concat-stream": "^1.5.2",
     "diacritics-map": "^0.1.0",
-    "gray-matter": "^2.1.0",
+    "gray-matter": "^3.1.1",
     "lazy-cache": "^2.0.2",
     "list-item": "^1.1.1",
     "markdown-link": "^0.1.1",