Fix the directory index test

Related to #104
Wouldn't something like "internally linking to folder-php/, but README.md missing" be better, to differentiate it from the case where the source link is directly to `folder-php/README.md` ?

And on a side-note, how does/would html-proofer cope with the situation where you have multiple directory_index_file options? http://httpd.apache.org/docs/2.2/mod/mod_dir.html#directoryindex