Avoid inter-lib, recursive nextTick warning on node 0.10

See #410 

Use `setImmediate` on node 0.10 to avoid hitting its max nested tick limit warning.

We've not had any direct reports from a when.js user who has encountered this.  However, the test case from RSVP is generic, so it's pretty clear that this could happen to any set of interleaving promise libs.

It's a shame that this requires a version check :(

cc @stefanpenner

Fix #410