fix compilation

msvc liked the previous code but travis said:
usr/include/boost/optional/optional.hpp:526:5: candidate constructor not
      viable: no known conversion from 'const config::attribute_value::yes_no'
      to 'argument_type' (aka 'const bool &') for 1st argument.
    optional ( argument_type val ) : base(val) {}
I hope travis likes this version.

src/scripting/game_lua_kernel.cpp

@@ -1432,9 +1432,9 @@ namespace {
 		{ return result_type(); }
 		//Cannot use bool, the case above would catch the yes_no and true_false values.
 		result_type operator()(const config::attribute_value::yes_no & b) const
-		{ return b; }
+		{ return static_cast<bool>(b); }
 		result_type operator()(const config::attribute_value::true_false & b) const
-		{ return b; }
+		{ return static_cast<bool>(b); }
 	};
 }
 int game_lua_kernel::intf_end_level(lua_State *L)