Add active_support to the app generator

Because of this trace:
https://travis-ci.org/rspec/rspec-rails/jobs/120133080#L5065-L5080

Author: Sam Phippen

example_app_generator/generate_app.rb

@@ -13,6 +13,8 @@
 function_script_file = File.join(rspec_rails_repo_path, 'script/functions.sh')
 
 in_root do
+  prepend_to_file "Rakefile", "require 'active_support/all'"
+
   # Remove the existing rails version so we can properly use master or other
   # edge branches
   gsub_file 'Gemfile', /^.*\bgem 'rails.*$/, ''