Requires Susy 2

[pimschaaf]

Susy 3 appears to exclude the full() mixin, which is used on line 4 of gravity-forms-scss/scss/partials/_progress.scss which will throw on an error like:

Error: no mixin named full on line 4 of assets/sass/vendor/gravity-forms-scss/scss/partials/_progress.scss >> @include full();

Downgrading to Susy 2 fixes this issue. It might be nice to clarify the Susy 2 dependency explicitly in the readme.