Upgrade broccoli-rollup to ^2.0

[dgeb]

Rollup is used to build the named AMD packages. I was unsuccessful in my attempts to upgrade broccoli-rollup to ^2.0, but have left the updated settings commented out:

• https://github.com/glimmerjs/glimmer-build/blob/master/lib/build-vendor-package.js#L43-L57

• https://github.com/glimmerjs/glimmer-build/blob/master/lib/to-named-amd.js#L25-L40

@chadhietala as the author of broccoli-rollup, it would be great if you have time to take a look.