Upgrade php-parser with PHP 5.6 language feature compatibility

Upgrade to nikic/php-parser v1.0.0beta1 and implement support for:
 - The power (`**`) binary and assignment operator.
 - Used functions and constants
 - Constant scalar expressions
 - Partial compatibility with  variadic parameters / argument unpacking.

Although this patch currently does not support variadic parameters in query expressions, it allows for these language features to be supported elsewhere in the source code. As the entire file where the query functions are defined is parsed with PHP-Parser, prior to v1.0.0beta1 it did not support these language features and parsing any files which used them would result in an error.