Adds a dependency on signet. It's to be used for auth, but for now it sslconfig module is required

src/ruby/grpc.gemspec

@@ -22,6 +22,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'xray'
   s.add_dependency 'logging', '~> 1.8'
   s.add_dependency 'google-protobuf', '~> 3.0.0alpha.1.1'
+  s.add_dependency 'signet', '~> 0.5.1'
   s.add_dependency 'minitest', '~> 5.4'  # reqd for interop tests
 
   s.add_development_dependency 'bundler', '~> 1.7'