Remove GitHub-based dependencies

This project has dependencies on the `master` branches of several GitHub repositories, including those from third parties. This raises the risk of accidental (or, theoretically, deliberate) breakage. All the gems have released versions available, so we should use those instead.
I chose that version to keep the changes to a minimum. The commit we had pinned in `Gemfile.lock` was [a few hundred commits](https://github.com/plataformatec/simple_form/compare/v3.0.2...92a6a9) ahead of the latest 3.0.x release (and [5 commits behind](https://github.com/plataformatec/simple_form/compare/92a6a9...v3.0.2), for good measure). With this release candidate, there’s only [13 commits](https://github.com/plataformatec/simple_form/compare/92a6a9...v3.1.0.rc1) between them.

I agree that we shouldn’t be using release candidates unless we have to, but I wanted to make as little functional change in this pull request as I could. I think testing whether we can pull back to a 3.0.x version without breaking anything is a good idea for something to do later.