chore(package): update nyc to version 15.1.0

Closes #24

Author: greenkeeper[bot]

package.json

@@ -23,7 +23,7 @@
     "documentation": "^8.0.0",
     "fs-extra": "^6.0.0",
     "level-browserify": "^1.1.2",
-    "nyc": "^12.0.1",
+    "nyc": "^15.1.0",
     "standard": "^11.0.1",
     "tape": "^4.6.3"
   },