Updating Nokogiri

As per PR #7

Author: Caitlin Bales (MSFT)

microsoft_graph.gemspec

@@ -27,5 +27,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "simplecov", "~> 0.11.1"
   spec.add_development_dependency "webmock", "~> 1.22.6"
 
-  spec.add_dependency "nokogiri", "~> 1.6.8"
+  spec.add_dependency "nokogiri", "~> 1.8.0"
 end