Add file extension to main

Author: ctavan

package.json

@@ -15,7 +15,7 @@
     "type": "git",
     "url": "git://github.com/ctavan/node-getenv.git"
   },
-  "main": "./index",
+  "main": "index.js",
   "scripts": {
     "test": "make test"
   },