Replaced deprecated process.nextTick with setImmediate

process.nextTick was deprecated in Node v0.10.0 stable (ref http://blog.nodejs.org/2013/03/11/node-v0-10-0-stable/) in favour of [setImmediate](http://nodejs.org/api/timers.html#timers_setimmediate_callback_arg). 

On Node 0.10.33, using RSVP causes deprecation error: ```(node) warning: Recursive process.nextTick detected. This will break in the next version of node. Please use setImmediate for recursive deferral.```

This issue seems to appear and disappear under strange circumstances: 
* https://github.com/stefanpenner/ember-cli/issues/1859
* https://github.com/stefanpenner/ember-cli/issues/2658

This PR replaces process.nextTick with setImmediate. All tests pass.