Update version of ratom and libxml-ruby

I ran `bundle update ratom` to get a newer version of libxml-ruby
(2.6.0 up from 2.3.2) as I couldn't get 2.3.2 to compile on my Mac:

    ruby_xml_node.c: In function ‘rxml_node_to_s’:
    ruby_xml_node.c:622: error: dereferencing pointer to incomplete type
    ruby_xml_node.c:624: error: dereferencing pointer to incomplete type
    make: *** [ruby_xml_node.o] Error 1

    make failed, exit code 2

This is similar to [commit 26dc19 in vanilla-rb][26dc19].

[26dc19]: https://github.com/lazyatom/vanilla-rb/commit/26dc1936c3c7f238a06168517a0a9475d51e7ec2