Bug#849458: cannot upgrade imagemagick-6-common

[積丹尼 Dan Jacobson]

$ apt-cache policy imagemagick-6-common
imagemagick-6-common:
  Installed: 8:6.9.6.6+dfsg-1
  Candidate: 8:6.9.7.0+dfsg-1
  Version table:
     8:6.9.7.0+dfsg-1 990
        990 http://free.nchc.org.tw/debian experimental/main amd64 Packages
 *** 8:6.9.6.6+dfsg-1 500
        500 http://free.nchc.org.tw/debian unstable/main amd64 Packages
        100 /var/lib/dpkg/status