Update 00-intro.md

php55-intl is no longer available as a formula.

See this commit: 
https://github.com/Homebrew/homebrew-php/commit/911d13a8ab240e73318a2321be95e16a13dce38d

It is now included in abstract-php formula which is extended by all the default php installations. (eg. php53 / php56 )