Updated upb to latest version (C89).

Since this version of upb supports C89, all of the
extra compiler flags are no longer required.
diff --git a/ruby/ext/google/protobuf_c/map.c b/ruby/ext/google/protobuf_c/map.c
index 12e7a9d..3436e09 100644
--- a/ruby/ext/google/protobuf_c/map.c
+++ b/ruby/ext/google/protobuf_c/map.c
@@ -120,7 +120,7 @@
 }
 
 static void* value_memory(upb_value* v) {
-  return (void*)(&v->val.uint64);
+  return (void*)(&v->val);
 }
 
 // -----------------------------------------------------------------------------