Use exports.default if _default is not defined, fixes #64 for typescript on es2017 target

[fasterthanlime]

cf. this comment

I feel bad about this, #64 was merged after I confirmed it worked for me when in fact.. I was probably misled by the cache? Sorry about that 😞

(Note that this will break if you manually define a variable named _default that isn't your default export but that's just the price to pay for babel compat I suppose?)

[anaisbetts]

I merged the revert to unbork people, can you rebase this against master (revert the revert first, then cherry-pick this)?

[fasterthanlime]

@paulcbetts done