chore: enable caching for travis (#1215)
chore: enable caching for travis (#1215)

Configure Travis to cache Maven’s `.m2` directory.
w3c/epubcheck,w3c/epubcheck,IDPF/epubcheck,w3c/epubcheck,IDPF/epubcheck,IDPF/epubcheck,IDPF/epubcheck,IDPF/epubcheck