Include edge Rails in the build matrix

Highlighting an incompatibility with the new implementation of
`[ActiveSupport::OrderedOptions#dig]`.

[^1]: 5c15b586aa

mrsk.gemspec

@@ -19,4 +19,8 @@ Gem::Specification.new do |spec|
   spec.add_dependency "zeitwerk", "~> 2.5"
   spec.add_dependency "ed25519", "~> 1.2"
   spec.add_dependency "bcrypt_pbkdf", "~> 1.0"
+
+  spec.add_development_dependency "debug"
+  spec.add_development_dependency "mocha"
+  spec.add_development_dependency "railties"
 end