fix npm audit

`npm audit` was reporting vulnerabilities.

I ran `npm audit fix --force`. None of the breaking changes in `nyc`
seem to affect us, and coverage still works.

Author: kvakil

.gitignore

@@ -11,6 +11,7 @@ out/
 npm-debug.log
 node_modules/
 package-lock.json
+.nyc_output/
 
 # Generated by scripts/configure.js
 config.gypi

package.json

@@ -51,9 +51,9 @@
   },
   "homepage": "https://github.com/nodejs/llnode#readme",
   "devDependencies": {
-    "codecov": "^3.1.0",
-    "nyc": "^13.1.0",
     "clang-format": "1.2.4",
+    "codecov": "^3.1.0",
+    "nyc": "^15.1.0",
     "tape": "^4.4.0"
   },
   "nyc": {