Circle CI update

[austinabell]

Previously the CI included linting in the build process, which was annoying and misleading if the linter failed. I don't care if we don't merge this in, but it would be nice :)

This change separates linting into its own workflow.

[ansermino]

Good call!

(Note: If we wanted to separate it even further we could use golangci.org, its basically pre-configured CI for all the golangci tools. That way testing and formatting/linting happen simultaneously)