Building native extensions. This could take a while...
ERROR: Error installing rmagick:
ERROR: Failed to build gem native extension.
Can't install RMagick 2.13.2. Can't find Magick-config
That means the devel package is not installed, so you pick it up on Ubuntu > 12.04
sudo apt-get install libmagick++-dev
and you get complete stack
dpkg -l | grep imagema
ii imagemagick
ii imagemagick-common
ii libmagic1:i386
ii libmagick++-dev
......
If suddenly you get this error
That means the devel package is not installed, so you pick it up on Ubuntu > 12.04
and you get complete stack
based on this link