I executed

gem install rmagick

and got the following error:

checking for Magick-config... no
Can't install RMagick 2.5.0. Can't find Magick-config in [...]

I had to install the libmagick9-dev package with

apt-get install libmagick9-dev

to solve this problem.

thanks to http://www.semanticpool.de/debian-rmagick-install-cant-find-magick-config/