Fix NSP dependency (yeoman#237)

* Fix NSP dependency

- Removed NSP Dependency
- Updated Npm scripts

* Change `prepublish` task to `prepublishOnly`

- Suggested by @silvenon at yeoman#237

Author: UlisesGascon

package.json

@@ -20,7 +20,7 @@
   "scripts": {
     "test": "jest",
     "pretest": "eslint . --fix",
-    "prepublish": "nsp check"
+    "prepublishOnly": "yarn audit"
   },
   "engines": {
     "node": ">=4.0.0"
@@ -45,7 +45,6 @@
     "eslint-plugin-prettier": "^2.6.0",
     "jest": "^23.5.0",
     "jest-cli": "^23.5.0",
-    "nsp": "^3.2.1",
     "prettier": "^1.10.2",
     "yeoman-assert": "^3.1.0",
     "yeoman-test": "^1.7.0"