Change all require RMagick to rmagick

We created a deprecation warning when requiring "RMagick"

We were getting warnings when running tests

Our code required "RMagick" quite a bit.

This PR changed all the requires to "rmagick", as we recommend.

I just checked out master an ran `git grep -l require | xargs perl -pi -w -e 's/require .RMagick./require "rmagick"/g;' `