Rename immutable.js to index.js so it can be found by Google Closure Compiler.

Author: Jonathan Mosney

immutable.js renamed to index.js

@@ -12,7 +12,7 @@
   ],
   "author": "Raynos <[email protected]>",
   "repository": "git://github.com/Raynos/xtend.git",
-  "main": "immutable",
+  "main": "index.js",
   "scripts": {
     "test": "node test"
   },