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
Merge pull request #1446 from hypothesis/fix-sass-3-4

Fix responsive styles when running Sass >= 3.4.