fix: update rexml to 3.3.9 to fix libncurses.so.5: cannot open shared object file issue

Author: nicolastremblay1

codedeploy_agent.gemspec

@@ -20,6 +20,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency('aws-sdk-s3', '~> 1')
   spec.add_dependency('docopt', '~> 0.5.0')
   spec.add_dependency('concurrent-ruby', '~> 1.1.9')
+  spec.add_dependency('rexml', '~> 3.3.9')
 
   spec.add_development_dependency('rake', '~> 12.3.3')
   spec.add_development_dependency('rspec', '~> 3.2.0')