Update installing_nokogiri.md

Use the Homebrew `/usr/local/opt/<brewname>` convention so that the version numbers shouldn't get out of date. By using `homebrew/dupes`, we also have the advantage of removing the entire section where `libiconv` is built by source, and can use the same convention for all the libraries.

The remainder of the copy is probably fine, though noticeably it's quite old (12/2012 !!!). While it's still not an easy process (the `</sarcasm>` tag still stands :smile:), this should make it significantly more future-proof.