package.json: set required Node.js version to 0.8.x

package.json

@@ -16,6 +16,9 @@
         "type": "git",
         "url": "git://github.com/amber-smalltalk/amber.git#0.10.0"
     },
+    "engines": {
+        "node": "0.8.x"
+    },
     "bin": {
         "amber": "./bin/amber",
         "amberc": "./bin/amberc",