[pkg] Update nyc to version 15.0.0 (#188)

Author: greenkeeper[bot]

package.json

@@ -40,7 +40,7 @@
     "assume": "^2.2.0",
     "browserify": "^16.2.3",
     "mocha": "^6.1.4",
-    "nyc": "^14.0.0",
+    "nyc": "^15.0.0",
     "pre-commit": "^1.2.2",
     "sauce-browsers": "^2.0.0",
     "sauce-test": "^1.3.3",