Fix CSS selector parser when only the :not pseudo class

before:

```ruby
1.9.3-p385 :001 > Nokogiri::CSS.xpath_for ':not(.a)'
Nokogiri::CSS::SyntaxError: unexpected '.' after ':not('
        from /Users/ykzts/work/nokogiri/lib/nokogiri/css/parser_extras.rb:87:in `on_error'
        from (eval):3:in `_racc_do_parse_c'
        from (eval):3:in `do_parse'
        from /Users/ykzts/work/nokogiri/lib/nokogiri/css/parser_extras.rb:62:in `parse'
        from /Users/ykzts/work/nokogiri/lib/nokogiri/css/parser_extras.rb:79:in `xpath_for'
        from /Users/ykzts/work/nokogiri/lib/nokogiri/css.rb:23:in `xpath_for'
        from (irb):1
        from /Users/ykzts/.rvm/rubies/ruby-1.9.3-p385/bin/irb:16:in `<main>'
```

after:

```ruby
1.9.3-p385 :001 > Nokogiri::CSS.xpath_for ':not(.a)'
 => ["//*[not(contains(concat(' ', normalize-space(@class), ' '), ' a '))]"]
```