Update package.json

The pull request to update Babel `6.x.x` raises the old `ReferenceError: _ApiClient is not defined` error on fresh installation, with node `4.2.4`.

All tests are green and everything seems fine with node `5.6.0`. Think its time to upgrade... Babel 6 plays nicer with 5.x.x anyway.

see PR: #935

Author: andrewmclagan

package.json

@@ -187,6 +187,6 @@
     "webpack-hot-middleware": "^2.5.0"
   },
   "engines": {
-    "node": "4.1.1"
+    "node": "5.6.0"
   }
 }