Log Message:
Update php-xdebug to 2.5.4.

Mon, May 15, 2017 - xdebug 2.5.4

= Fixed bugs:

- Fixed issue #799: Function traces report base class instead of object name
- Fixed issue #1421: Fix set_time_limit hanging on PHP 5.6 when pcntl_exec
  does not exist (Frode E. Moe)
- Fixed issue #1429: Code coverage does not cover null coalesce
- Fixed issue #1434: Code coverage segfaults on 32-bit arch