When you run tests or rake, if you see:
WARNING: Nokogiri was built against LibXML version 2.9.0, but has dynamically loaded 2.7.8
Then do the following:
gem uninstall nokogiri libxml-ruby brew update brew uninstall libxml2 brew install libxml2 --with-xml2-config brew uninstall libxslt brew install libxslt brew unlink libxslt bundle config build.nokogiri -- --with-xml2-dir=/usr --with-xslt-dir=/usr --with-iconv-dir=/usr bundle
Nokogiri should now be compiled against the right version!