Bump cross-spawn and webpack-cli

Bumps [cross-spawn](https://github.com/moxystudio/node-cross-spawn) and [webpack-cli](https://github.com/webpack/webpack-cli). These dependencies needed to be updated together.

Updates `cross-spawn` from 6.0.5 to 6.0.6
- [Changelog](https://github.com/moxystudio/node-cross-spawn/blob/v6.0.6/CHANGELOG.md)
- [Commits](moxystudio/node-cross-spawn@v6.0.5...v6.0.6)

Updates `webpack-cli` from 3.3.9 to 3.3.12
- [Release notes](https://github.com/webpack/webpack-cli/releases)
- [Changelog](https://github.com/webpack/webpack-cli/blob/master/CHANGELOG.md)
- [Commits](webpack/webpack-cli@v3.3.9...v3.3.12)

---
updated-dependencies:
- dependency-name: cross-spawn
  dependency-type: indirect
- dependency-name: webpack-cli
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

yarn.lock

@@ -2119,9 +2119,9 @@ cross-env@^5.1:
     cross-spawn "^6.0.5"
 
 cross-spawn@^6.0.0, cross-spawn@^6.0.5:
-  version "6.0.5"
-  resolved "https://registry.yarnpkg.com/cross-spawn/-/cross-spawn-6.0.5.tgz#4a5ec7c64dfae22c3a14124dbacdee846d80cbc4"
-  integrity sha512-eTVLrBSt7fjbDygz805pMnstIs2VTBNkRm0qxZd+M7A5XDdxVRWO5MxGBXZhjY4cqLYLdtrGqRf8mBPmzwSpWQ==
+  version "6.0.6"
+  resolved "https://registry.yarnpkg.com/cross-spawn/-/cross-spawn-6.0.6.tgz#30d0efa0712ddb7eb5a76e1e8721bffafa6b5d57"
+  integrity sha512-VqCUuhcd1iB+dsv8gxPttb5iZh/D0iubSP21g36KXdEuf6I5JiioesUVjpCdHV9MZRUfVFlvwtIUyPfxo5trtw==
   dependencies:
     nice-try "^1.0.4"
     path-key "^2.0.1"