added force option in electron-download.

Downloading from cache causes a checksum check that causes some builds to fail(see https://github.com/electron/electron/issues/8653) that  that rely on stable electron releases(pre electron v1.7.0 beta) where chromedrive is not named based on electron version. This will add the option to always force a redownload.

npm/install.js

@@ -29,6 +29,7 @@ download({
   platform: process.env.npm_config_platform,
   arch: process.env.npm_config_arch,
   strictSSL: process.env.npm_config_strict_ssl === 'true',
+  force: process.env.force_no_cache || false,
   quiet: ['info', 'verbose', 'silly', 'http'].indexOf(process.env.npm_config_loglevel) === -1
 }, extractFile)