Update version of Docker used in CircleCI build

[mikesir87]

With the merge of #55, the CircleCI build needs to use a later version of Docker. In order to do so, it looks like the config might need to be bumped to use the 2.0 format.

[nettofarah]

Thanks for filling the issue :)
I might take a look at this sometime soon, but feel free to go ahead and give it a try if you feel like it

[NicolasRitouet]

I've proposed a PR #60 to use the new circleci config