deps: update gyp to 25ed9ac

This is a port of a525c724 .

Original commit message:

  Gyp update to be able to generate VS2015 projects.

  PR-URL: https://github.com/joyent/node/pull/25857


Reviewed-By: Alexis Campailla <alexis@janeasystems.com>

PR-URL: https://github.com/nodejs/node/pull/2843


Reviewed-By: rvagg - Rod Vagg <rod@vagg.org>
Reviewed-By: orangemocha - Alexis Campailla <orangemocha@nodejs.org>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>