Add Asciidoctor EPUB3, set prerelease gem versions

* Add the Asciidoctor EPUB3 gem
* set the version of any prerelease gem so we get the right one
Add Asciidoctor EPUB3; general revisions
Add required packages for installing Nokogiri
Install nokogiri gem using system libraries
Install epubcheck and kindlegen gems for Asciidoctor EPUB3
Install nokogiri gem from RPM
Rearrange some lines
Fix version of asciidoctor-epub3 gem
Install slim gem for custom templates
Merge pull request #5 from hasalex/master

Install slim gem for custom templates
Add tar installation