Update dev-dependencies

Author: wooorm

package.json

@@ -25,7 +25,7 @@
     "mdast-comment-config": "^1.0.0",
     "mdast-github": "^1.0.0",
     "mdast-lint": "^1.0.0",
-    "mdast-slug": "^1.0.0",
+    "mdast-slug": "^2.0.0",
     "mdast-validate-links": "^1.0.0",
     "mocha": "^2.0.0"
   },