Add the rexml dependency explictly

[wata727]

See also aws/aws-sdk-ruby#2459 (comment)

Ruby 3.0 build is failing because rexml is now bundled gem and aws-sdk-ruby requires to add XML parser dependency explicitly.

https://github.com/wata727/elastic_whenever/runs/1712170050

An error occurred while loading ./spec/schedule_spec.rb.
Failure/Error: require "aws-sdk-ecs"

RuntimeError:
  Unable to find a compatible xml library. Ensure that you have installed or added to your Gemfile one of ox, oga, libxml, nokogiri or rexml
# ./vendor/bundle/ruby/3.0.0/gems/aws-sdk-core-3.111.1/lib/aws-sdk-core/xml/parser.rb:74:in `set_default_engine'