Update to the latest nokogiri dependency when possible

Author: pitbulk

ruby-saml.gemspec

@@ -39,8 +39,10 @@ Gem::Specification.new do |s|
     s.add_runtime_dependency('nokogiri', '<= 1.5.11')
   elsif RUBY_VERSION < '2.1'
     s.add_runtime_dependency('nokogiri', '>= 1.5.10', '<= 1.6.8.1')
+  elseif RUBY_VERSION < '2.3'
+    s.add_runtime_dependency('nokogiri', '>= 1.9.1', '<= 1.10.0')
   else
-    s.add_runtime_dependency('nokogiri', '>= 1.5.10')
+    s.add_runtime_dependency('nokogiri', '>= 1.10.5')
   end
 
   s.add_development_dependency('coveralls')