Disable Buble transforms not needed for Node 4

[Conduitry]

Many of the transforms being done by Buble aren't required for Node 4. Disabling them is nicer, and also slightly decreases the size of the bundle - down from 385KB to 375KB.

[Rich-Harris]

Good catch. We can actually get exactly the same result by using target: { node: 4 }, not sure why I didn't do that in the first place — added a commit. Slightly more futureproof (if we were to start using other ES6 features that Buble supports and Node 4 doesn't), and easier to drop support for Node 4 when the time comes.

[Swatinem]

drop support for Node 4

soon…