always tripple slash file uris

this isn't too awesome because it results in file:////foo on posix, which
is a bit meh but meaningwise the same as file:///file. on windows however
it ensures file:///C:/foo which is the only way the uri is valid (needs
the thrid slash to parse as path since windows paths do not start with
one already)
long term this had better been written as File.join, but for the time
being this is the quicker and reliable fix