use different ImageMagick library?

Met @maxim the other day after giving [a talk](https://docs.google.com/presentation/d/1vuXqmMOssslx3nRxqWh1e3SgGN7_euYdVMIWm94DDPA/edit) about magickly, and he suggested using [Skeptic](https://github.com/maxim/skeptick) as the ImageMagick (which I'll call "IM") wrapper.

I haven't been actively working on magickly in a while, and haven't been following progress on IM Ruby libraries. The issue with Dragonfly 0.x (not sure about 1.x) is that each transformation is applied in a separate `convert` command
- Use [the `single-convert` changes](https://github.com/afeld/magickly/compare/single-convert) done by @tolsen forever ago
- Would want to do some cleanup/documentation before merging in – see https://github.com/afeld/magickly/issues/16
- Ideally this logic would be at the IM wrapper level, keeping magickly simple
- Use RMagick
- All in memory (not sure if this is a good or bad thing)
- Read veeery mixed reviews about it when first building magickly and deciding what to use (e.g. memory leaks), though this may have improved since
- Use Skeptic
- From a quick glance, seems well-thought-out, and well-suited to keeping magickly easily extensible.
- The DSL could make the existing [shortcuts](https://github.com/afeld/magickly/blob/master/lib/shortcuts.rb) a lot more readable.
- [Doesn't do argument format enforcement](https://github.com/maxim/skeptick#security-note) – ideally this would be outside magickly as well
- Only runs a single `convert` command (correct @maxim?), so theoretically should have the same performance characteristics as `single-convert`

Feedback/ideas welcome!

P.S. This change would obviate https://github.com/afeld/magickly/issues/48.