I was getting the same error and it got resolved after installing the libmagick9-dev library on ubuntu 11.
Update (Thanks to the commenters – Dec. 2012)
$ sudo apt-get install libmagickwand-dev or $ sudo apt-get install graphicsmagick-libmagick-dev-compat for ubuntu 12
$ sudo apt-get install libmagickwand-dev or $ sudo apt-get install libmagick9-dev for ubuntu 11 or below.
$ yum install ImageMagick-devel for centos
$ brew install imagemagick for Mac OS (using Homebrew)