Incorrectly removes CSS variables

Example:

```
.wrapper { --border: black; }
#navigation div { border: 1px solid var(--border) }
```

In this case, the variable can still be valid if `#navigation` is inside `.wrapper`. However, clean-css assumes that it's invalid and outputs `.wrapper{--border:#000}#navigation div{border:1px solid}`, which is wrong. Oddly enough, it seems to leave other variable uses alone.
