gitweb: hard-depend on the Digest::MD5 5.8 module

Since my d48b2841 ("perl: bump the required Perl version to 5.8 from
5.6.[21]", 2010-09-24), we've depended on 5.8, so there's no reason to
conditionally require Digest::MD5 anymore. It was released with perl
v5.7.3[1]

The initial introduction of the dependency in
e9fdd74e ("gitweb: (gr)avatar support", 2009-06-30) says as much,
this also undoes part of the later 2e9c8789 ("gitweb: Mention
optional Perl modules in INSTALL", 2011-02-04) since gitweb will
always be run on at least 5.8, so there's no need to mention
Digest::MD5 as a required module in the documentation, let's instead
say that we require perl 5.8.

1. $ corelist Digest::MD5
   Data for 2015-02-14
   Digest::MD5 was first released with perl v5.7.3

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>