cwltool missing python dep html5lib

## Expected Behavior

All dependencies should be installed via pip

## Actual Behavior

Following docs [here](https://github.com/common-workflow-language/cwltool#install) to install just cwltool to virtual env results in the following warning when running `cwltool --validate my.cwl` (running under travis):

```
/home/travis/build/cancerit/dockstore-cgpmap/venv/lib/python3.4/site-packages/rdflib/plugins/parsers/structureddata.py:30: UserWarning: html5lib not found! RDFa and Microdata parsers will not be available.
```

Link to line in travis output [here](https://travis-ci.org/cancerit/dockstore-cgpmap/builds/346698962#L2756).

## Workflow Code
Not sure relevant but [here](https://github.com/cancerit/dockstore-cgpmap/blob/639ef8a22e7030ed61929884aafef930cf641910/Dockstore.cwl).

## Full Traceback
NA

## Your Environment
* cwltool version: 1.0.20180224135035
* Link to travis section: [here](https://travis-ci.org/cancerit/dockstore-cgpmap/builds/346698962#L2751-L2767)

Installed under virtualenv with [this travis config](https://github.com/cancerit/dockstore-cgpmap/blob/443a1a8579c51cb8981140bb865e99cfd8f8bcf3/.travis.yml).

Resolved via [this travis change](https://github.com/cancerit/dockstore-cgpmap/compare/443a1a8579c51cb8981140bb865e99cfd8f8bcf3...639ef8a22e7030ed61929884aafef930cf641910) for pip.