Fix broken rsvp ref for correct mime type...

Original responds now with text/plain, updated version will respond with application/javascript.

Original no longer works (not sure if there are other errors as well) on http://addyosmani.github.io/basket.js/ on any browser I tried.

Refused to execute script from 'https://raw.github.com/tildeio/rsvp.js/1.0.0/browser/rsvp.min.js' because its MIME type ('text/plain') is not executable, and strict MIME type checking is enabled. addyosmani.github.io/basket.js/:1
Uncaught ReferenceError: RSVP is not defined basket.min.js:10
Uncaught ReferenceError: basket is not defined