Display host name on failure

Sample output.

```
  4) Port "80" should be listening
     On host `centos`
     Failure/Error: it { should be_listening }
       sudo netstat -tunl | grep -- :80\
       expected Port "80" to be listening
     # ./spec/centos/httpd_spec.rb:13:in `block (2 levels) in <top (required)>'
```