Update nyc to the latest version 🚀 (#69)

* chore(package): update nyc to version 15.0.0

* upgrade babel-plugin-istanbul

Co-authored-by: Renée Kooi <renee@kooi.me>

package.json

@@ -25,14 +25,14 @@
     "@babel/core": "^7.0.0",
     "@babel/preset-env": "^7.0.0",
     "@babel/register": "^7.0.0",
-    "babel-plugin-istanbul": "^5.1.2",
+    "babel-plugin-istanbul": "^6.0.0",
     "chai": "^4.1.2",
     "cross-env": "^6.0.0",
     "eslint": "^6.0.0",
     "eslint-config-airbnb-base": "^14.0.0",
     "eslint-plugin-import": "^2.18.2",
     "mocha": "^6.0.0",
-    "nyc": "^14.0.0",
+    "nyc": "^15.0.0",
     "rollup": "^1.0.0",
     "rollup-plugin-babel": "^4.0.2"
   }