chore: update nyc to v15

No change to logic. This updates nyc (code coverage tool) to version 15.
The main goal is to work around a bug on the latest nodejs LTS versions.

This change indicates it only supports nodejs >= 8.9. Hopefully this is
good enough for our purposes since ShellJS currently supports
nodejs >= 8.

Fixes #1130

package.json

@@ -66,13 +66,12 @@
     "eslint-config-airbnb-base": "^13.1.0",
     "eslint-plugin-import": "^2.17.3",
     "js-yaml": "^3.14.1",
-    "nyc": "^14.1.1",
+    "nyc": "^15.1.0",
     "shelljs-changelog": "^0.2.6",
     "shelljs-release": "^0.5.2",
     "shx": "^0.3.4",
     "travis-check-changes": "^0.4.0"
   },
-  "optionalDependencies": {},
   "engines": {
     "node": ">=8"
   }