updates asciidoctor gem to 1.5.0.preview.7
adds better way to test CLI command.
resolves issue #157 by adding two methods for registering a Ruby class from java and also for adding a require command to Ruby runtime. In that last case, the required gem should be available to load path (by using classpath approach or by setting during AsciidoctorJ construction.
resolves issue #152 by adding CLI options added on version 1.5.0.preview7