[test] update nyc to v15 to fix tests on windows

nyc@<15 is broken on Windows (see istanbuljs/nyc#1205)

Author: raphinesse

package.json

@@ -76,7 +76,7 @@
     "eslint-plugin-import": "2.x",
     "linklocal": "^2.8.2",
     "mocha": "^3.5.3",
-    "nyc": "^11.9.0",
+    "nyc": "^15.0.0",
     "redux": "^3.7.2",
     "rimraf": "^2.7.1",
     "semver": "^6.3.0",