Skip md5_file() for files the webserver can't read

Aegir installs a drushrc.php to the sites dir which is not readable to the www-data user.

As a result, archimedes_directory_hash() reports this for sites reporting from Archimedes under Aegir.

Warning: md5_file(/var/aegir/platforms/stacks-7.14/sites/example.org/drushrc.php): failed to open stream: Permission denied in archimedes_directory_hash() (line 578 of /var/aegir/platforms/stacks-7.14/sites/all/libraries/archimedes/archimedes.class.php).