Update rails requirement from ~> 4.0 to ~> 5.2

Updates the requirements on [rails](https://github.com/rails/rails) to permit the latest version.
- [Release notes](https://github.com/rails/rails/releases)
- [Commits](https://github.com/rails/rails/commits/v5.2.1)

Signed-off-by: dependabot[bot] <support@dependabot.com>

dotenv-rails-safe.gemspec

@@ -39,7 +39,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler", "~> 1.13"
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_development_dependency "rspec", "~> 3.0"
-  spec.add_development_dependency "rails", "~> 4.0"
+  spec.add_development_dependency "rails", "~> 5.2"
   spec.add_development_dependency "coveralls"
   spec.add_development_dependency "pry"
   spec.add_development_dependency "bump"