Use `string` method to stringify token if `literal` method is not provided.

'literal' method is not implemented by Quote::Double or Quote::Interpolate.
<http://search.cpan.org/%7Eadamk/PPI-1.215/lib/PPI/Token/Quote.pm#literal>
Merge pull request #14 from ishikawa/fix-missing-method-error-in-rename-class-inheritance

Fix missing 'literal' method error while renaming class