fix 'uninitialized constant ActiveSupport::Autoload' in some machines

error:
number_helper.rb:3:in `<module:NumberHelper>': uninitialized constant ActiveSupport::Autoload (NameError)

related issues:
https://github.com/rails/rails/issues/14664
https://github.com/beeminder/beeminder-gem/issues/10
https://github.com/nulldb/nulldb/issues/38
Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.