support redcarpet 3.1.0

because ArgumentError occurred:

    % redcarpet -v
    Redcarpet 3.1.0
    % echo "# title" | md2hiki
    /.../lib/ruby/gems/2.1.0/gems/md2hiki-0.0.1/lib/redcarpet/render/hiki.rb:48:in `header': wrong number of arguments (3 for 2) (ArgumentError)
            from /.../lib/ruby/gems/2.1.0/gems/md2hiki-0.0.1/bin/md2hiki:45:in `render'
            from /.../lib/ruby/gems/2.1.0/gems/md2hiki-0.0.1/bin/md2hiki:45:in `<top (required)>'
            from /.../bin/md2hiki:23:in `load'
            from /.../bin/md2hiki:23:in `<main>'

see https://github.com/vmg/redcarpet/commit/facd5c5aa1c228a68a336d0989bae35dcc020f7f