fix(package): update npm to version 5.5.1

Closes #113

Author: greenkeeper[bot]

package.json

@@ -27,7 +27,7 @@
     "js-yaml": "^3.3.1",
     "lodash": "^4.16.4",
     "nopt": "^4.0.0",
-    "npm": "^4.0.3",
+    "npm": "^5.5.1",
     "npm-registry-client": "^7.3.0",
     "npmlog": "^4.0.0",
     "parse-git-config": "^1.1.1",