Set global variable explicitly

Fixes:

```
DEPRECATION WARNING on line 85 of test/test.scss:
Assigning to global variable "$mq-breakpoints" by default is deprecated.
In future versions of Sass, this will create a new local variable.
If you want to assign to the global variable, use "$mq-breakpoints: mq-add-breakpoint(cinema, 4096px) !global" instead.
```