This installation of RMagick was configured with ImageMagick 6.5.5 but ImageMagick 6.5.7-8 is in use.
Uninstall “rmagick” gem
$ sudo gem uninstall rmagick
Compile and install RMagick from source code
$ wget http://files.rubyforge.vm.bytemark.co.uk/rmagick/RMagick-2.13.1.tar.gz
$ tar xzf RMagick-2.13.1.tar.gz
$ cd RMagick-2.13.1
$ ruby setup.rb
$ sudo ruby setup.rb install
Test and see if it works
$ irb
irb(main):001:0> require "RMagick"
=> true
https://bugs.launchpad.net/ubuntu/source/librmagick-ruby/bug/565461