Update package.json

package.json

@@ -12,6 +12,7 @@
   },
 
   "main": "./lib/base58",
+  "engine": "node > 0.4.0 < 0.7.0",
 
   "scripts": {
     "test": "make test"
@@ -19,7 +20,7 @@
 
   "devDependencies": {
     "coffee-script": "1.3.1",
-    "mocha": "*",
-    "should": "*"
+    "mocha": "1.0.x",
+    "should": "0.6.x"
   }
 }