chore(package): update nyc to version 15.0.1

Closes #17

Author: greenkeeper[bot]

package.json

@@ -26,7 +26,7 @@
 		"jsdoc-to-markdown": "^4.0.1",
 		"mocha": "^5.2.0",
 		"mocha-junit-reporter": "^1.17.0",
-		"nyc": "^12.0.2",
+		"nyc": "^15.0.1",
 		"rewire": "^4.0.1",
 		"sinon": "^6.0.0",
 		"sinon-chai": "^3.1.0",