hypothesis/h — pull request #1446 by aron (2014-08-25 15:27:23+00:00)
Title: Fix responsive styles when running Sass >= 3.4

In Sass 3.4 the index() function now returns null instead of false. So we just perform a truthy check in the respond-to mixin rather than a direct comparison.

http://sass-lang.com/documentation/file.SASS_CHANGELOG.html

@gergely-ujvari should fix your issue.