Bump nth-check and gulp-imagemin

Bumps [nth-check](https://github.com/fb55/nth-check) and [gulp-imagemin](https://github.com/sindresorhus/gulp-imagemin). These dependencies needed to be updated together.

Updates `nth-check` from 1.0.2 to 2.1.1
- [Release notes](https://github.com/fb55/nth-check/releases)
- [Commits](https://github.com/fb55/nth-check/compare/v1.0.2...v2.1.1)

Updates `gulp-imagemin` from 7.1.0 to 8.0.0
- [Release notes](https://github.com/sindresorhus/gulp-imagemin/releases)
- [Commits](https://github.com/sindresorhus/gulp-imagemin/compare/v7.1.0...v8.0.0)

---
updated-dependencies:
- dependency-name: nth-check
  dependency-type: indirect
- dependency-name: gulp-imagemin
  dependency-type: direct:development
...

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

package.json

@@ -56,7 +56,7 @@
         "gulp-autoprefixer": "^8.0.0",
         "gulp-babel": "^8.0.0",
         "gulp-if": "^3.0.0",
-        "gulp-imagemin": "^7.1.0",
+        "gulp-imagemin": "^8.0.0",
         "gulp-load-plugins": "^2.0.8",
         "gulp-merge": "^0.1.1",
         "gulp-plumber": "^1.2.1",