Add nokogiri minimum version

cometsong · web-flow · commit b93d6af6e097 · 2019-06-11T13:31:05.000-04:00
As suggested by a Github security alert.
diff --git a/Gemfile b/Gemfile
@@ -1,2 +1,3 @@
 source 'https://rubygems.org'
 gem 'github-pages'
+gem "nokogiri", ">= 1.8.5"
