Fix missing 'literal' method error while renaming class

This fixes _"Can't locate object method "literal" via package "PPI::Token::Quote::Double""_ error while renaming class name in `use base "..."` (and `use base qq{...}`).

```perl
package Child {
    use base "Parent";
}
```

The `literal` method of PPI::Token::Quote is not implemented by ::Quote::Double or ::Quote::Interpolate ([POD](http://search.cpan.org/%7Eadamk/PPI-1.215/lib/PPI/Token/Quote.pm#literal)). So I use the `string` method if the `literal` method is missing.