Add activesupport as runtime dependency

it is required to run commands such as `.blank?`. Previously it was
being installed upstream in the deprecated case_transform gem

threema.gemspec

@@ -21,6 +21,7 @@ Gem::Specification.new do |spec|
   spec.files         = Dir['{lib}/**/*']
   spec.require_paths = ['lib']
 
+  spec.add_runtime_dependency 'activesupport', '~> 7.0.8'
   spec.add_runtime_dependency 'dotenv'
   spec.add_runtime_dependency 'mimemagic'
   spec.add_runtime_dependency 'mime-types'