Fix for LibSass error compilining keyframe @mixin

Fix for libsass error for echoing out vars

More information here : https://github.com/sindresorhus/grunt-sass/issues/33
Merge branch 'master' of https://github.com/oller/animate-sass into oller-master

Conflicts:
	helpers/_mixins.scss -- FIXED
Merge branch 'oller-master'