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/[email protected]: 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"
 }