Don't require runtime dependency on rexml, not needed
in Ruby 2.5-2.7 where it is in stdlib.

Index: .metadata
--- .metadata.orig
+++ .metadata
@@ -17,7 +17,7 @@ dependencies:
     - - ">="
       - !ruby/object:Gem::Version
         version: '0'
-  type: :runtime
+  type: :development
   prerelease: false
   version_requirements: !ruby/object:Gem::Requirement
     requirements: