fix(package.json): loosen the engines/npm semver range to prevent fal…

…se warnings

This should get rid the following warning when installing rxjs:


```
npm WARN engine @reactivex/rxjs@5.0.0-alpha.7: wanted: {"npm":"~2.0.0"} (current: {"node":"4.2.1","npm":"2.14.7"})
```

package.json

@@ -109,7 +109,7 @@
  "watch": "0.16.0"
  },
  "engines": {
- "npm": "~2.0.0"
+ "npm": ">=2.0.0"
  },
  "typings": "./dist/cjs/Rx.d.ts"
 }