Libxml-ruby aren't compatible with libxml2 2.9.x until version 2.4.0.
To keep developing legacy proyects, you must install older version of libxml2 with homebrew.
First check versions availables:
brew versions libxml2
Right now, the older version compatible is 2.7.6 so:
git checkout 8939a91 Library/Formula/libxml2.rb
brew install libxml2
ln -s /usr/local/opt/libxml2/bin/xml2-config /usr/local/bin/xml2-config
Now you can bundle install or gem install older versions of libxml-ruby