Remove RMagick Requirement

[merbjedi]

Just use imagemagick command line calls (via exec), similar to how paperclip and mini_magick work.

[aliang]

If you don't remove the RMagick requirement, I think the require line

require 'rmagick'

is case-sensitive, in lib/image_combiner.rb

At least, it was on my Linux installation. I had to manually change it to

require 'RMagick'