Node.js: used distinct placeholder for version in "package.json".

This makes the "sed" instruction simpler and more portable, as the previous
variant didn't work well on BSD systems due to the "\s" metacharacter.

Thanks to Sergey A. Osokin <osa@FreeBSD.org.ru> for spotting this issue.

Also, this should prevent accidentally creating a version 1.0.0 package.

src/nodejs/unit-http/package.json

@@ -1,6 +1,6 @@
 {
     "name": "unit-http",
-    "version": "1.0.0",
+    "version": "%%VERSION%%",
     "description": "HTTP module for NGINX Unit",
     "main": "http.js",
     "files": [