Change package to point at build/jssm.es5.js

[StoneCypher]

So

If you're packaging your own downstream, from node which today does not have import, this is what you want

If you're packaging the es6 yourself, then the src copy is what you want

If you're pulling for a browser, and/or from a CDN, the cjs copy in dist is what you want

This fixes the issue in #290 as a result

This seems to be principle of least surprise? It's a hard call. A document section will be needed. This will need to change, probably invoking a major, in the import era.

[StoneCypher]

i should maybe declare a major here? but i'm not very confident this will stay