publisher: use new-style class declaration

[diegodelemos]

• Closes status publishing exception reana-workflow-engine-yadage#98.

[tiborsimko]

Works for me with:

$ reana-dev git-status -s
- reana-workflow-engine-serial @ master 4b94287 tasks: decoding json values passed from WFC
 M Dockerfile
?? reana-commons/
- reana-workflow-engine-cwl @ master 699cdee tasks: decoding json values passed from WFC
 M Dockerfile
?? reana-commons/
- reana-db @ master 1705ee6 release: 0.5.0.dev20190116
- reana-message-broker @ master 7f18a21 release: v0.4.0
- reana-server @ master d8ce77e global: workflow execution scheduler
 M Dockerfile
?? reana-commons/
- reana-workflow-monitor @ master c242d4f release: v0.4.0
- reana-workflow-controller @ master d54cf79 global: restructure workflow run managers code
 M Dockerfile
?? reana-commons/
- reana-workflow-engine-yadage @ master 4d082a1 tasks: decoding json values passed from WFC
 M Dockerfile
?? reana-commons/
- reana-commons @ pr-80 2552392 publisher: use new-style class declaration
- reana-job-controller @ master 83f3764 installation: bump reana-commons version
 M Dockerfile
?? reana-commons/
- pytest-reana @ master c661adf release: 0.5.0.dev20190116

Please prepare release commit and release on PyPI.