maxbrunsfeld/counterfeiter v6.0.0 on GitHub

This release is a rewrite of counterfeiter that supports go modules (#94).

counterfeiter is now a go module
counterfeiter will now be released using the semver versioning scheme
any regression in behavior from v5 will be considered a bug in v6, unless this regression is related to go 1.9 or earlier
going forward, we will support N-1 (e.g. today, go 1.11.1 is the latest version of go, so go 1.10 and go 1.11 are supported); this matches the go team's support policy and thus our use of golang.org/x/tools/go/packages