CON-2020: upgrade CSS Janus lib

* use composer and fetch the latest version of PHP port of CSS Janus
* get rid of old copy from ``/lib`` and use the one installed by composer

Fix the following CSS:

```css
.foo {
  box-shadow: 1px 2px 3px 4px
}
```

Is now correctly transformed to:

```css
.foo {
  box-shadow: -1px 2px 3px 4px
}
```

No need to add unit tests as the library itself uses test cases from CSSJanus - https://github.com/cssjanus/cssjanus/blob/master/test/data.json#L239