Optmize build process installing govendor only if necessary
Optmize build process installing govendor only if necessary

When executing the build process twice, we end up trying to install
`govendor` every build.

```
$ make build
>> fetching govendor
>> fetching dependencies

$ make build
>> fetching govendor
>> fetching dependencies
```

This commit introduces a rule to install govendor when necessary, so
first time contributors will have the tool installed and then it will
not try to install again.

```
$ make build
>> fetching govendor
>> fetching dependencies

$ make build
>> fetching dependencies
```

digitalocean/do-agent,digitalocean/do-agent