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)
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