Leave the space after media query `not`

The CSS fragment

``` css
@media only screen and not (max-width:768px) {
```

gets minified as

``` css
@media only screen and not(max-width:768px){
```

However that is an error and does not work in Firefox. It should become:

``` css
@media only screen and not (max-width:768px){
```

--madmurphy