Merge pull request #567 from tbetbetbe/grpc_ruby_rename_modules

src/ruby/ext/grpc/rb_byte_buffer.c

src/ruby/ext/grpc/rb_byte_buffer.h

src/ruby/ext/grpc/rb_call.c

src/ruby/ext/grpc/rb_call.h

src/ruby/ext/grpc/rb_channel.c

src/ruby/ext/grpc/rb_channel.h

src/ruby/ext/grpc/rb_completion_queue.c

src/ruby/ext/grpc/rb_completion_queue.h

src/ruby/ext/grpc/rb_credentials.c

src/ruby/ext/grpc/rb_credentials.h

src/ruby/ext/grpc/rb_event.c

src/ruby/ext/grpc/rb_event.h

src/ruby/ext/grpc/rb_grpc.c

src/ruby/ext/grpc/rb_grpc.h

src/ruby/ext/grpc/rb_metadata.c

src/ruby/ext/grpc/rb_metadata.h

src/ruby/ext/grpc/rb_server.c

src/ruby/ext/grpc/rb_server.h

src/ruby/ext/grpc/rb_server_credentials.c

src/ruby/ext/grpc/rb_server_credentials.h