fix problem with PPI::Token::Number::Hex->literal() and upper-case 'X' on perl < 5.14

My PPI post-1.215 change to support upper-case 'X' in hex numbers did not cover PPI::Token::Number::Hex->literal(), which relies on being able to call hex() on the string representation and get the numeric equivalent. perl < 5.14 requires 'x', not 'X', so needed to support that.

There would have been a similar problem with PPI::Token::Number::Binary->literal(), except that it takes advantage--probably inadvertently--of the fact that "B" evaluates to 0 in multiplication.