GitLab

Created by: rriemann

I used gollum to push this wiki markdown to the gitlab instance:

```yml
  ## Backup settings
  backup:
    # path: "tmp/backups"   
    keep_time: 604800   # default: 0 (forever) (in seconds)
```

I would expect that a fallback of no highlighting at all would be used instead of raising an error.

The page could not be rendered. production.log gives:

  Completed 500 Internal Server Error in 1908ms

  ActionView::Template::Error (Traceback (most recent call last):
    File "/home/git/gitlab/vendor/bundle/ruby/1.9.1/gems/gitlab-pygments.rb-0.3.2/lib/pygments/mentos.py", line
  303, in start
      res = self.get_data(method, lexer, args, kwargs, text)
    File "/home/git/gitlab/vendor/bundle/ruby/1.9.1/gems/gitlab-pygments.rb-0.3.2/lib/pygments/mentos.py", line
  171, in get_data
      res = self.highlight_text(text, lexer, formatter_name, args, _convert_keys(opts))
    File "/home/git/gitlab/vendor/bundle/ruby/1.9.1/gems/gitlab-pygments.rb-0.3.2/lib/pygments/mentos.py", line
  122, in highlight_text
      lexer = self.return_lexer(lexer, args, kwargs, code)
    File "/home/git/gitlab/vendor/bundle/ruby/1.9.1/gems/gitlab-pygments.rb-0.3.2/lib/pygments/mentos.py", line 79, in return_lexer
      return lexers.get_lexer_by_name(lexer, **inputs)
    File "/home/git/gitlab/vendor/bundle/ruby/1.9.1/gems/gitlab-pygments.rb-0.3.2/vendor/pygments-main/pygments/lexers/__init__.py", line 80, in get_lexer_by_name
      raise ClassNotFound('no lexer for alias %r found' % _alias)
  ClassNotFound: no lexer for alias 'yml' found
  ):
      11: .file_holder
      12:   .file_content.wiki
      13:     = preserve do
      14:       = render_wiki_content(@wiki)
      15: 
      16: - commit = CommitDecorator.new(@wiki.version)
      17: %p.time Last edited by #{commit.author_link(avatar: true, size: 16)} #{time_ago_in_words @wiki.created_at} ago
    lib/redcarpet/render/gitlab_html.rb:25:in `block_code'
    app/helpers/gitlab_markdown_helper.rb:50:in `render'
    app/helpers/gitlab_markdown_helper.rb:50:in `markdown'
    app/helpers/gitlab_markdown_helper.rb:55:in `render_wiki_content'
    app/views/wikis/show.html.haml:14:in `block in _app_views_wikis_show_html_haml__547832907_100044020'
    app/views/wikis/show.html.haml:13:in `_app_views_wikis_show_html_haml__547832907_100044020'
    app/controllers/wikis_controller.rb:15:in `show'