Update Sass MQ 1.3 to 2.0-rc.1

- [new] Use new [awesome Sass maps](http://viget.com/extend/sass-maps-are-awesome) for breakpoints configuration
- [change] Renamed function `mq-retrieve-breakpoint-width` into `mq-get-breakpoint-width`
- [fix] No more warnings from tests when compiling our Sass files (removed the test/ directory of sass-mq)

See https://github.com/guardian/sass-mq/releases/tag/v2.0.0-rc.1 for a detailed view of changes.

![ ](http://24.media.tumblr.com/7f535b33ff29f392957d1247704ea21b/tumblr_mvtu14zwpd1s2yegdo1_400.gif)