[R-sig-Debian] New libpng in c2d4u3.5 breaks many other packages

[Jeroen Ooms]

As of today many Travis users are seeing this error:

The following packages have unmet dependencies:
 libmagick++-dev : Depends: libmagickcore-dev (=
8:6.7.7.10-6ubuntu3.9) but it is not going to be installed
                   Depends: libmagickwand-dev (=
8:6.7.7.10-6ubuntu3.9) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.


The problem is that a few hours ago a version of libpng1.6 was
published to c2d4u3.5 which is not compatible with the version of
imagemagick in trusty. Becuase libmagick++ is installed by default on
Travis, this breaks almost all installations.

I think it's best to only publish R packages in c2d4u, not system libs.