Support rpm on be_installed` matcher

This pull request following test.

```
describe package('httpd') do
  it { should be_installed.by('rpm').with_version('2.2.15-28.el6') }
end
```

until apply this patch, following result.

```
 1) Package "httpd" 
     Failure/Error: it { should be_installed.by('rpm').with_version('2.2.15-28.el6') }
     ArgumentError: `be_installed` matcher doesn't support rpm
       
       `be_installed` matcher doesn't support rpm
```