Set 2.5 as minimum required ruby version for gem

This gem is no longer tested with Rubies older than 2.5,
and it's actually broken on at least <= 2.2.

By setting the minimum version in the `gemspec`, we ensure that older Ruby
versions don't try to use an incompatible `rexml` version.

Issue #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 = '>= 2.5.0'
+
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "test-unit"