ldc spec: use known-good commit from upstream repo

The test harness uses a public D-lang project to test
whether the ldc compiler in fedora works. The project
is currently broken on master branch and fails to compile:

    dhcptest.d(435): Error: std.range.chunks(Source)(Source source,
    size_t chunkSize) at /usr/include/d/std/range.d(6517) conflicts
    with std.stdio.chunks at /usr/include/d/std/stdio.d(2397)

    /usr/include/d/std/range.d(6517): Error: template instance Chunks!(string)
    does not match template declaration Chunks(Source)
    if (isInputRange!(Source) && hasSlicing!(Source) && hasLength!(Source))

    dhcptest.d(435): Error: template instance std.range.chunks!(string)
    error instantiating

Therefore, checkout the 0.4 git tag, which *does* compile
and should never change.