Changed task.rendere_opts arg to backend_name: :latex
- Errors:
  $ be rake test
  rake aborted!
  ArgumentError: wrong number of arguments (1 for 0)
  /Users/carlson/Dropbox/prog/git/asciidoctor-latex/Rakefile:98:in `block in <top (required)>'
  /Users/carlson/Dropbox/prog/git/asciidoctor-latex/Rakefile:95:in `new'
  /Users/carlson/Dropbox/prog/git/asciidoctor-latex/Rakefile:95:in `<top (required)>'