Update content/installing_nokogiri.md

adding step to use gcc-4.2 when installing nokogiri with homebrew 0.9 and osx mountain lion, 
as reported by brax444 (https://github.com/sparklemotion/nokogiri/issues/442#issuecomment-7978408)
Merge pull request #16 from seltzered/patch-1

Update content/installing_nokogiri.md