Error installing nokogiri: Failed to build gem native extension & libiconv is missing (OSX)

Run these commands:

gem uninstall nokogiri
xcode-select --install
gem install nokogiri

source: http://www.nokogiri.org/tutorials/installing_nokogiri.html#mac_os_x

sometimes mac updates can break xcode CLI so reinstalling can fix the issue:
https://github.com/sparklemotion/nokogiri/issues/1445