Support older ruby 2.1.

README.md

@@ -172,7 +172,7 @@ Querying a document using a namespace:
 
 | Ruby     | Required      | Recommended |
 |:---------|:--------------|:------------|
-| MRI      | >= 2.3.0      | >= 2.6.0    |
+| MRI      | >= 1.9.3      | >= 2.1.2    |
 | JRuby    | >= 1.7        | >= 1.7.12   |
 | Rubinius | Not supported |             |
 | Maglev   | Not supported |             |

oga.gemspec

@@ -29,7 +29,7 @@ Gem::Specification.new do |s|
     s.extensions = ['ext/c/extconf.rb']
   end
 
-  s.required_ruby_version = '>= 2.3.0'
+  s.required_ruby_version = '>= 1.9.3'
 
   s.add_dependency 'ast'
   s.add_dependency 'ruby-ll', '~> 2.1'