Removed 'optional' from proto3 syntax file.

ruby/src/main/sentinel.proto

@@ -3,13 +3,13 @@ package com.google.protobuf.jruby;
 option optimize_for = CODE_SIZE;
 
 message Sentinel {
-  optional int32 default_int32 = 1;
-  optional int64 default_int64 = 2;
-  optional uint32 default_unit32 = 3;
-  optional uint64 default_uint64 = 4;
-  optional string default_string = 5;
-  optional bool default_bool = 6;
-  optional float default_float = 7;
-  optional double default_double = 8;
-  optional bytes default_bytes = 9;
+  int32 default_int32 = 1;
+  int64 default_int64 = 2;
+  uint32 default_unit32 = 3;
+  uint64 default_uint64 = 4;
+  string default_string = 5;
+  bool default_bool = 6;
+  float default_float = 7;
+  double default_double = 8;
+  bytes default_bytes = 9;
 }