tools: update gyp to 25ed9ac

Includes improved support for VS 2015[0] and makes it possible to build
with ninja again[1].

[0] https://codereview.chromium.org/1112753003
[1] https://codereview.chromium.org/1209553002

Fixes: https://github.com/nodejs/io.js/pull/2065
PR-URL: https://github.com/nodejs/io.js/pull/2074


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>