Remove Gemfile.lock from the repo as it's bad practice for gems.

Do not check your Gemfile.lock into version control, since it enforces precision that does not exist in the gem command, which is used to install gems in practice. Even if the precision could be enforced, you wouldn’t want it, since it would prevent people from using your library with versions of its dependencies that are different from the ones you used to develop the gem.
Remove redundant VERSION file.

This is managed in lib/machinist/version.rb
Regenerate the gem skeleton with Bundler.
Fix RSpec syntax deprecations.
Ignore .rbx artefacts.
Test with multiple versions of ActiveRecord.