List facter as a dependence, used by QPS test

grpc.gemspec

@@ -35,6 +35,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'googleauth',      '~> 0.5.1'
 
   s.add_development_dependency 'bundler',            '~> 1.9'
+  s.add_development_dependency 'facter',             '~> 2.4'
   s.add_development_dependency 'logging',            '~> 2.0'
   s.add_development_dependency 'simplecov',          '~> 0.9'
   s.add_development_dependency 'rake',               '~> 10.4'