go get fails

```
➜  go git:(master) ✗ go get -u gopkg.in/matm/v1/gocov-html/cmd/gocov-html
# github.com/matm/gocov-html/pkg/cov
XXX/.gvm/pkgsets/go1.11/global/src/github.com/matm/gocov-html/pkg/cov/report.go:161:17: undefined: io.ReadAll
```