Failed to go build -o wsk using README.md instructions due to older client-go version

I cannot get the wsk CLI to build, following the README.md instructions page.

The `go build -o wsk` command fails with the following error -
```
go build -o wsk
# github.com/apache/incubator-openwhisk-cli/commands
commands/action.go:911:10: limits.Concurrency undefined (type *"github.com/apache/incubator-openwhisk-cli/vendor/github.com/apache/incubator-openwhisk-client-go/whisk".Limits has no field or method Concurrency)
commands/project.go:40:20: cannot use cobraCMD (type *"github.com/apache/incubator-openwhisk-cli/vendor/github.com/spf13/cobra".Command) as type *"github.com/spf13/cobra".Command in argument to cmd.Deploy
commands/project.go:51:22: cannot use cobraCMD (type *"github.com/apache/incubator-openwhisk-cli/vendor/github.com/spf13/cobra".Command) as type *"github.com/spf13/cobra".Command in argument to cmd.Undeploy
commands/project.go:63:20: cannot use cobraCMD (type *"github.com/apache/incubator-openwhisk-cli/vendor/github.com/spf13/cobra".Command) as type *"github.com/spf13/cobra".Command in argument to cmd.Deploy
commands/project.go:74:26: cannot use cobraCMD (type *"github.com/apache/incubator-openwhisk-cli/vendor/github.com/spf13/cobra".Command) as type *"github.com/spf13/cobra".Command in argument to cmd.ExportCmdImp
```

I can resolve the `cobra` errors by deleting the package from the vendor dir (not ideal), but the other issue,

```
go build -o wsk
# github.com/apache/incubator-openwhisk-cli/commands
commands/action.go:911:10: limits.Concurrency undefined (type *"github.com/apache/incubator-openwhisk-cli/vendor/github.com/apache/incubator-openwhisk-client-go/whisk".Limits has no field or method Concurrency)
```
, seems to be stemming from a mismatched client-go version.

I can see the master branch for github.com/apache/incubator-openwhisk-client-go/ now has the `Concurrency` field added to the `Limits` struct (https://github.com/apache/incubator-openwhisk-client-go/blob/master/whisk/shared.go#L100), but `go get` seems to fetch an older commit of the client-go library causing the build to fail.