Set 2.6 as minimum required ruby version for gem

Ruby 2.6 is the oldest Ruby that hasn't reached EOL.
By setting it in the `gemspec`, we ensure that older Ruby
versions don't try to use an incompatible `rexml` version.

Issue ruby#69

rexml.gemspec

@@ -55,6 +55,8 @@ Gem::Specification.new do |spec|
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
 
+  spec.required_ruby_version = '>= 2.6.0'
+
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "test-unit"