Fix CHANGELOG generation in preversion script now that an npm env var changed

Author: papandreou

package.json

@@ -69,7 +69,7 @@
     "test": "mocha",
     "test:ci": "npm run coverage",
     "coverage": "NODE_ENV=development nyc --reporter=lcov --reporter=text --all -- mocha && echo google-chrome coverage/lcov-report/index.html",
-    "preversion": "offline-github-changelog --next=${npm_package_version} > CHANGELOG.md && git add CHANGELOG.md"
+    "preversion": "offline-github-changelog --next=${npm_new_version} > CHANGELOG.md && git add CHANGELOG.md"
   },
   "nyc": {
     "include": [