chore(package): update nyc to version 15.1.0

Closes #10

Author: greenkeeper[bot]

package.json

@@ -58,6 +58,6 @@
     "ava": "1.3.0",
     "coveralls": "3.0.3",
     "execa": "1.0.0",
-    "nyc": "13.3.0"
+    "nyc": "15.1.0"
   }
 }