Package: Updates libsass verion.

* Locking down v2.0.0.
Related Issue: #602.

package.json

 {
   "name": "node-sass",
   "version": "2.0.0-beta",
-  "libsass": "3.1.0-beta",
+  "libsass": "3.1.0",
   "description": "Wrapper around libsass",
   "license": "MIT",
   "homepage": "https://github.com/sass/node-sass",