Breaks on Sass 3.3 "!global" syntax

In Sass 3.3, to set a global variable you do this:

```
$someVariable: true !global;
```

If you attempt to compile this file with libsass, it fails and you get this error:

```
error reading values after true
```

It appears that libsass does not recognize the new `!global` modifier coming up in Sass 3.3.
