Update Nokogiri version for CVE-2019-5477 (#154)

* Update Nokogiri version for CVE-2019-5477

refs: sparklemotion/nokogiri#1915

* Fix build errors in Ruby 2.1 and 2.2

Author: sinsoku

wovnrb.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'activesupport'
   spec.add_dependency 'addressable'
   spec.add_dependency 'lz4-ruby'
-  spec.add_dependency 'nokogiri', '~> 1.8.1'
+  spec.add_dependency 'nokogiri', '>= 1.8.1'
   spec.add_dependency 'nokogumbo', '>= 1.4.0', '< 2.0.0'
   spec.add_dependency 'rack'