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.
allow to install docs since this is a dev environment
rebuild and bump from fedora 20 to fedora 21

Note: git-remote-hg obsoletes git-hg as of
https://bugzilla.redhat.com/show_bug.cgi?id=1170008
and resolves failure to install git-hg:

    Error: Package: git-hg-2.1.0-2.fc21.noarch (fedora)
    Requires: git = 2.1.0-2.fc21
    Available: git-2.1.0-2.fc21.x86_64 (fedora)
               git = 2.1.0-2.fc21
    Installing: git-2.1.0-4.fc21.x86_64 (updates)
               git = 2.1.0-4.fc21

See also https://admin.fedoraproject.org/updates/git-2.1.0-4.fc21