Removes unused dependencies

src/ruby/grpc.gemspec

@@ -21,14 +21,10 @@ Gem::Specification.new do |s|
   s.require_paths = ['lib']
   s.platform      = Gem::Platform::RUBY
 
-  s.add_dependency 'faraday', '~> 0.9'
   s.add_dependency 'google-protobuf', '~> 3.0.0alpha.1.1'
   s.add_dependency 'googleauth', '~> 0.1'
   s.add_dependency 'logging', '~> 1.8'
-  s.add_dependency 'jwt', '~> 1.2.1'
   s.add_dependency 'minitest', '~> 5.4'  # reqd for interop tests
-  s.add_dependency 'multi_json', '1.10.1'
-  s.add_dependency 'signet', '~> 0.6.0'
   s.add_dependency 'xray', '~> 1.1'
 
   s.add_development_dependency 'bundler', '~> 1.7'