Attempting to upgrade aws-lambda-go version in a project with go.mod fails

```
alex@penguin /t/x> cat go.mod 
module foo

go 1.12

require github.com/aws/aws-lambda-go v1.8.1
alex@penguin /t/x> go get -u github.com/aws/aws-lambda-go
go: gopkg.in/urfave/cli.v1@v1.21.0: go.mod has non-....v1 module path "github.com/urfave/cli" at revision v1.21.0
go get: error loading module requirements
```

