Support TypeScript 2.6

[OliverJAsh]

TypeScript 2.6 is now in RC. https://blogs.msdn.microsoft.com/typescript/2017/10/12/announcing-typescript-2-6-rc/

I'm not sure if we need to change anything in the parser, but at the very least we can update the supported version notice.

[JamesHenry]

Yep, seems all good thanks @OliverJAsh!

I have opened a PR so you can start using that if you wish, I will keep it synced with master as usual.