build: make node-gyp output silent

As it is, node-gyp produces a lot of build related verbose messages.
Latest node-gyp upgrade allows us to specify --silent flag to suppress
those messages. Except for CI, addons build will run silently.

PR-URL: https://github.com/nodejs/node/pull/8990



Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>