package.json: update 'markdownlint-cli' requirement to v0.15.0

package.json

@@ -4,7 +4,7 @@
   "description": "Simplified, community-driven man pages",
   "dependencies": {
     "glob": "7.1.3",
-    "markdownlint-cli": "0.1.0",
+    "markdownlint-cli": "0.15.0",
     "tldr-lint": "~0.0.7",
     "husky": "^2.2.0"
   },