Use render to get the rendered CSS.

`Ember.View.prototype.renderToBuffer` was private API and is removed on
current master.

This swaps to using all public API surface, and works properly on all
three Ember channels (stable, beta, and canary).  It should work all the
way back to 1.0 (though I didn't test that far).
[changed] Ember compatibility range
release v2.0.0