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

Author: papandreou

package.json

@@ -17,7 +17,7 @@
     "generate-site": "generate-site --require ./bootstrap-unexpected-markdown.js",
     "update-examples": "generate-site --require ./bootstrap-unexpected-markdown.js --update-examples",
     "deploy-site": "deploy-site.sh",
-    "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"
   },
   "repository": {
     "type": "git",