add package dependency libffi-dev for ruby 2.2.0
add package dependency libffi-dev for ruby 2.2.0

https://twitter.com/hsbt/status/548085823430602753
wata-gh/itamae-plugin-recipe-rtn_rbenv,namusyaka/itamae-plugin-recipe-rtn_rbenv,rutan/itamae-plugin-recipe-rtn_rbenv