Merge pull request #7 from tbetbetbe/core_creds_api_change

src/ruby/bin/apis/pubsub_demo.rb

src/ruby/bin/math_client.rb

src/ruby/bin/noproto_client.rb

src/ruby/ext/grpc/rb_channel.c

src/ruby/ext/grpc/rb_channel_credentials.c

src/ruby/ext/grpc/rb_channel_credentials.h

src/ruby/ext/grpc/rb_grpc.c

src/ruby/lib/grpc/generic/client_stub.rb

src/ruby/pb/test/client.rb

src/ruby/spec/channel_credentials_spec.rb

src/ruby/spec/channel_spec.rb

src/ruby/spec/client_server_spec.rb

src/ruby/spec/generic/client_stub_spec.rb