updates asciidoctor version to 1.5.1.
resolves #207 by excluding asciidoctor gem from epub dependency.
[maven-release-plugin] prepare release asciidoctorj-1.5.1
[maven-release-plugin] prepare for next development iteration
promotes parse_only_header option to Options class.
Merge pull request #208 from lordofthejars/master

Prepare pom.xml for release of Asciidoctorj 1.5.1