Update to RSVP 3.0.6 - some optimizations in place

[harningt]

Some of the change log entries for RSVP (that this is based on) indicate optimizations being put in place. Would it be practical to update es6-promise to that? I see that they added RSVP.Promise - though I'm not certain how/if that would be a conflict or benefit.

[jakearchibald]

@stefanpenner is looking into making this lib a build step of RSVP (take RSVP.Promise and remove the non-standard bits).

Is there a particular optimisation you're keen on? I'll look into rolling that in earlier.

[harningt]

No rush in updating (on an external library "freeze" right now).
Also - at the moment there's no particular optimization that I found - just in the release notes for 3.0.0 of RSVP, there was a mention of

• Significant internal cleanup and performance improvements