gitignore: adding .vs/ directory to .gitignore

This is created by vs 2015 for user & machine-specific files and should
be ignored by git.

PR-URL: https://github.com/nodejs/node/pull/6070


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>