epubcheck 4

References epubcheck:4.0.0-alpha12 (see IDPF/epubcheck#471) and its dependencies. Also enables the EPUB3 Validator script, which should work after this is merged.

Depends on pull requests:
daisy/pipeline-scripts-utils#59
daisy/pipeline-scripts#54

I wasn't quite sure where to list these dependencies. The build doesn't seem to pull in transitive dependencies so I specified them here so that they are included in the resulting build.