correct deprecated "enable-iri" arg

Small fix to correct `with-iri` arg behavior.

Currently, `brew install wget --with-iri` still builds `wget` with the `--disable-iri` arg; as seen below.

    $ brew install wget --with-iri
    ==> Downloading http://ftpmirror.gnu.org/wget/wget-1.16.tar.xz
    Already downloaded: /Library/Caches/Homebrew/wget-1.16.tar.xz
    ==> ./configure --prefix=/usr/local/Cellar/wget/1.16 --sysconfdir=/usr/local/etc --with-ssl=openssl --with-libssl-prefix=/usr/local/opt/openssl --disable-iri