deps: enable node-gyp iojs.lib download checksum

Originally disabled in commit 5de334c ("deps: make node-gyp work again
on windows") due to the then-website lacking the requisite SHASUMS.txt
or SHASUMS256.txt files.  The website has a SHASUMS256.txt now so start
checksumming the download again.

PR-URL: nodejs/node#918
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>