add RGSS_VERSION (RGSS3)

Even though 3.0.1 is the current version, 3.0.0 is used, because mkxp doesn't
implement the changes like not raising a TypeError for 'Color.new == nil' yet.
3.0.1 (finally) returns false. Other changes don't affect mkxp (I think).

See <http://tkool.jp/support/download/rpgvxace/rpgvxace_update> (Japanese).

binding-mri/binding-mri.cpp

@@ -113,6 +113,8 @@ static void mriBindingInit()
 
 		_rb_define_module_function(rb_mKernel, "msgbox",    mriPrint);
 		_rb_define_module_function(rb_mKernel, "msgbox_p",  mriP);
+
+		rb_define_global_const("RGSS_VERSION", rb_str_new_cstr("3.0.0"));
 	}
 	else
 	{