Remove sirupsen/logrus from promrus' vendor.

```
$ rm -fr vendor/github.com/weaveworks/promrus/vendor/github.com/sirupsen
```

This fixes build errors like:
```
vendor/github.com/weaveworks/common/logging/logging.go:35: cannot use hook (type *promrus.PrometheusHook) as type "github.com/weaveworks/scope/vendor/github.com/sirupsen/logrus".Hook in argument to "github.com/weaveworks/scope/vendor/github.com/sirupsen/logrus".AddHook:
    *promrus.PrometheusHook does not implement "github.com/weaveworks/scope/vendor/github.com/sirupsen/logrus".Hook (wrong type for Fire method)
        have Fire(*"github.com/weaveworks/scope/vendor/github.com/weaveworks/promrus/vendor/github.com/sirupsen/logrus".Entry) error
        want Fire(*"github.com/weaveworks/scope/vendor/github.com/sirupsen/logrus".Entry) error
```