process: changed var to const in internal/process/promises

PR-URL: https://github.com/nodejs/node/pull/8620


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jackson Tian <shvyo1987@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>