Add imagemagick to basic setup apt-get install.

Imagemagick is mentioned in the "first steps" section, but it's not actually listed anywhere in an `apt-get install` statements, which makes it easy to forget about, so I added it to the last such statement at the end of the basic setup section.
Merge pull request #2488 from toolness/patch-3

Add imagemagick to basic setup apt-get install.