Add require for hydra?

Thanks for your hard work on this project. I packaged this recently for the Guix package manager, which generally expects packages to be built with their own set of dependencies, so that if packages A and B each depends on package C, then package A can update to use a newer version of C without any impact on B.

On Guix, I noticed that without requiring hydra, the package wouldn't work properly; it's not enough to add `:after hydra` in the smerge defhydra. I patched in the require for Guix; I didn't have any problem when using the require-less package using a version not managed by Guix.

Would you be willing to add in a require statement?