cwl:cwlVersion in RDF output has incorrect base URL

## Expected Behavior
This value should be either a literal or URL with a base of w3id.org/cwl or commonwl.org
```
a cwl:Workflow ;
cwl:cwlVersion ;
```

## Actual Behavior
Actual output uses the base URL of the workflow:
```
a cwl:Workflow ;
cwl:cwlVersion ;
```

## Command to reproduce
```
cwltool --print-rdf https://raw.githubusercontent.com/common-workflow-language/workflows/master/workflows/lobSTR/lobSTR-workflow.cwl
```

## Cwltool version
* 1.0.20170525215327