Fix rsvp at v 2.0.4, workaround Travis failure.

At v3.0, rsvp no longer contains lib/, so we fall over.
I opened https://github.com/tildeio/rsvp.js/issues/182.
and https://github.com/google/traceur-compiler/issues/618

TBR=johnjbarton

Review URL: https://github.com/google/traceur-compiler/pull/619

Closes #619.