Remove rmagick dependency

Is it necessary to include both Dragonfly and Rmagick? Rmagick is only being used in a single place when getting the image dimensions after an asset upload. I've change that code to use Dragonfly instead. I try to stay away from Rmagick because it isnt actively supported. It took months before it was updated to work with imagemagick v6.8.*

There is a gotcha:
See this commit: https://github.com/Sharagoz/engine/commit/d23abdbb59f64300fab928d0d1b9a4313e72f24b
The identify command seems to have been accidentally mapped to the convert command in the dragonfly initializer. The identify command has not been used until now, so this has been undetected. People who upgrade to Locomotive v2.0 regenerate the initializers and should not be affected, but people who have already upgaraded will run into issues when uploading images if they dont fix the initializer.