media-gfx/imagemagick: update to 6.9.7.4

Previous version of ImageMagick (6.7.8.7) uses an older version of pnglib
(1.5) and has problems with loading certain type of images with
chromaticities map. This is a reason why it was generating warnings.

BUG=chromium:717247
TEST=Run graphics_Sanity test with new version of ImageMagick.\
Conversion warning disappered

Change-Id: I613798fcb3a9a689bc4c0ba8611d8aba980721de
Signed-off-by: Robert Tarasov <tutankhamen@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/497171
Commit-Ready: Ilja H. Friedel <ihf@chromium.org>
Reviewed-by: Ilja H. Friedel <ihf@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
4 files changed