imagemagick: security bump to version 7.0.7-39

>From the release notes:

2018-06-06  7.0.7-39  <quetzlzacatenango@image...>
  * Fixed numerous use of uninitialized values, integer overflow, memory
    exceeded, and timeouts (credit to OSS Fuzz).

The most critical of these are:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=8772
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=8782



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>