Update nyc to v13.1.0

Closes postcss#220

Author: RyanZim

package.json

@@ -36,7 +36,7 @@
     "coveralls": "^3.0.0",
     "eslint": "^5.0.0",
     "eslint-config-problems": "1.1.0",
-    "nyc": "^11.0.2",
+    "nyc": "^13.1.0",
     "postcss-import": "^12.0.0",
     "prettier": "~1.14.0",
     "sugarss": "^2.0.0",