Require activesupport in Rakefile

The guide says this is needed for cherry-picking just the string
extensions, and without it our specs fail.

Cf: https://guides.rubyonrails.org/active_support_core_extensions.html#loading-grouped-core-extensions

Rakefile

@@ -12,6 +12,7 @@ RSpec::Core::RakeTask.new(:spec)
 
 task default: [:test_setup, :spec]
 
+require "active_support"
 require "active_support/core_ext/string"
 
 desc "Setup test app"