Add a global flag on quote variables

Sass 3.3 is now [out](https://twitter.com/SassCSS/status/442125990248988672).
This PR fix the warning about declaring a scoped variable without the `!global` flag.

```
DEPRECATION WARNING on line 8 of base/_quotes.scss:
Assigning to global variable "$open-quote" by default is deprecated.
In future versions of Sass, this will create a new local variable.
If you want to assign to the global variable, use "$open-quote: \201C !global" instead.
Note that this will be incompatible with Sass 3.2.
```