Upgrade minor/major versions of dependencies that don't require code …

…changes

The changelogs of those dependencies showed no breaking changes for us.
Most of the time the major version bump was done to remove compatibility
with very outdated Node.js versions.

Only for `autoprixer` and `gulp-postcss` a change was required, which is
including `postcss` in our `package.json` explicitly since it's now a
peer dependency of those packages.

Now only `acorn`,`systemjs`, `terser` and `yargs` are not the latest
versions because they require more work.