Use autoprefixer for postcss instead of gulp-autoprefixer

Use `autoprefixer` for postcss
(https://www.npmjs.com/package/autoprefixer) instead of
`gulp-autoprefixer`.

Changes:
- Use postcss and autoprefixer in the gulp sass tasks
- By using `autoprefixer` directly, `autoprefixer` version is updated
from `5.0` to the latest version `6.5.3`.

Changes in the CSS:
- Improve the `transition: transform…` and `transition: box-shadow…`
support

Full diff of `foundation.css`: https://www.diffchecker.com/iMxTO2UH