fix MRUBY_VERSION value

RUBY_ENGINE_VERSION and MRUBY_VERSION should refer to the same string.

src/version.c

@@ -9,7 +9,7 @@ mrb_init_version(mrb_state* mrb)
   mrb_define_global_const(mrb, "RUBY_VERSION", mrb_str_new_lit(mrb, MRUBY_RUBY_VERSION));
   mrb_define_global_const(mrb, "RUBY_ENGINE", mrb_str_new_lit(mrb, MRUBY_RUBY_ENGINE));
   mrb_define_global_const(mrb, "RUBY_ENGINE_VERSION", mruby_version);
-  mrb_define_global_const(mrb, "MRUBY_VERSION", mrb_str_new_lit(mrb, MRUBY_VERSION));
+  mrb_define_global_const(mrb, "MRUBY_VERSION", mruby_version);
   mrb_define_global_const(mrb, "MRUBY_RELEASE_NO", mrb_fixnum_value(MRUBY_RELEASE_NO));
   mrb_define_global_const(mrb, "MRUBY_RELEASE_DATE", mrb_str_new_lit(mrb, MRUBY_RELEASE_DATE));
   mrb_define_global_const(mrb, "MRUBY_DESCRIPTION", mrb_str_new_lit(mrb, MRUBY_DESCRIPTION));