Add support for PHP 7.1

[vincentchalamon]

PHP 7.1 is stable since December 1st, and gives a lot of enhancements. I.e., it allows to add a ? on method return type to allow null or type (?string).

php7cc ran on a PHP7.1 project throw following errors:

> Syntax error, unexpected '?' on line 22. Processing aborted.

[sstalle]

It's the same issue as #79. php7cc needs to be upgraded to use PHP-Parser 3. Unfortunately I can't dedicate much time to this project now and I can't give an ETA, but I'm open to pull requests.

[vincentchalamon]

Duplicate from #79