[DRE-maint] ruby-rmagick is marked for autoremoval from testing

[Debian testing autoremoval watch]

ruby-rmagick 2.16.0-2 is marked for autoremoval from testing on 2018-03-21

It is affected by these RC bugs:
888193: ruby-rmagick: FTBFS on ruby2.5: <FrozenError(<can't modify frozen Magick::Image>)>