The gem detects natively installed libxml and uses instead of installing new one from rubygems. There might be some errors connected to this. If you get this error concering libxml, like
checking for libxml/xmlversion.h in /opt/include/libxml2,/opt/local/include/libxml2,/usr/local/include/libxml2,/usr/include/libxml2... no
*** extconf.rb failed ***
or
mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h
then you might not have installed libxml for ruby. I.e. in debian something like
sudo aptitude install ruby-libxml
or using equivalent command in other package managers.
If you want to use libxml from rubygems, than uninstall the native one or add this line to your Gemfile
'libxml-ruby', '3.0'