Bump shelljs from 0.7.8 to 0.8.5

Bumps [shelljs](https://github.com/shelljs/shelljs) from 0.7.8 to 0.8.5.
- [Release notes](https://github.com/shelljs/shelljs/releases)
- [Changelog](https://github.com/shelljs/shelljs/blob/master/CHANGELOG.md)
- [Commits](shelljs/shelljs@v0.7.8...v0.8.5)

---
updated-dependencies:
- dependency-name: shelljs
  dependency-type: direct:development
...

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

Author: dependabot[bot]

package-lock.json

@@ -45,7 +45,7 @@
     "postcss-url": "^7.2.1",
     "rimraf": "^2.6.0",
     "semver": "^5.3.0",
-    "shelljs": "^0.7.6",
+    "shelljs": "^0.8.5",
     "stylus": "^0.54.5",
     "stylus-loader": "^3.0.2",
     "uglifyjs-webpack-plugin": "^1.1.1",