Cannot run site with `java -jar` due to asciidoctorj error

- [ ] @lordofthejars (2/13) to respond to https://github.com/spring-io/sagan/pull/191#issuecomment-34979293
- [x] @gregturn to work with @lordofthejars per https://github.com/spring-io/sagan/issues/191#issuecomment-31614365
- [x] @gregturn to determine why Asciidoctor factory creation fails, i.e. what is the stack trace we’re seeing in https://github.com/spring-io/sagan/issues/191#issuecomment-30402519 all about?

- [ ] Remove the _**Note**_ from the [run the site locally](https://github.com/spring-io/sagan/wiki/Run-the-site-locally) wiki page when this issue is fixed.

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Changes introduced with commit d83e63ff0aea4a7302dade5e46558454d73259e5 cause the following exception on startup with `java -jar sagan-site/build/libs/sagan-site.jar`: 

    org.jruby.exceptions.RaiseException: (LoadError) no such file to load -- asciidoctor

See https://gist.github.com/anonymous/7925440 for complete stack trace.