Prevent from ASDF read error when the character encoding is ASCII

[fukamachi]

README.md containing UTF-8 characters is read in jonathan.asd and it causes read error while loading:

Error while trying to load definition for system jonathan from pathname
/srv/www/fukabot/bundle-libs/software/jonathan-20151218-git/jonathan.asd:
   READ error during LOAD:

     :ASCII stream decoding error on
     #<SB-SYS:FD-STREAM
       for "file /srv/www/fukabot/bundle-libs/software/jonathan-20151218-git/README.md"
       {100D8B0353}>:

       the octet sequence #(227) cannot be decoded.

     (in form starting at line: 6, column: 0, position: 95)

[rudolph-miller]

Thanks!