Add rexml as an explicit dependency for Ruby 3.x

[postmodern]

• rexml was removed as a default gem from Ruby 3.x, so we need to add it as an explicit dependency.
• Restrict the rexml version family to 3.0+. Ruby 2.6 shipped with rexml-3.1.9.1 so this should be compatible.

[postmodern]

The rexml dependency could be bumped to ~> 3.1 since Ruby 2.3.0 shipped with 3.1.7.3. https://docs.ruby-lang.org/en/2.3.0/REXML.html