Add 2.3 as minimum required ruby version for gem

Ruby 2.3 is the oldest Ruby that still passes all tests when running
`rake test`. By setting it in the `gemfile`, we ensure that older Ruby
versions don't try to use an incompatible `rexml` version.

(Note: It's probably trivial to support the older Rubies, but I'll
leave that for someone who is interested enough in doing the extra
work)

Issue #69

Author: ivoanjo

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 = '>= 2.3.0'
+
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "test-unit"