Port CircleCI config to v2 format

[PeterJCLaw]

Hi,

I'm raising an issue in case you're not aware -- Circle CI are sunsetting their 1.0 configuration format (which this repo uses) in favour of their 2.0 format at the end of August 2018. More details can be found in their documentation at https://circleci.com/sunset1-0/

Thanks,