remove engine declaration

this repo obviously works with engines newer then what was specified. This just results in an extra noisy warning

```js
npm WARN engine [email protected]: wanted: {"node":"0.6.x"} (current: {"node":"1.6.2","npm":"2.7.3"})
```

package.json

@@ -13,6 +13,5 @@
  "scripts": { "test": "NODE_PATH=./lib mocha --ui exports" },
  "devDependencies": {
  "mocha": "0.12.x"
- },
- "engines": { "node": "0.6.x" }
+ }
 }