lib,install: always download SHA sums on Windows
lib,install: always download SHA sums on Windows

PR-URL: https://github.com/nodejs/node-gyp/pull/1926
Reviewed-By: João Reis <5095b819f0ac7660978fa44706cfa411a9116ec5@janeasystems.com>
Reviewed-By: Richard Lau <8193d74c06330af7778ef55510160e4e561ed6a1@uk.ibm.com>

nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodegit/node-gyp