fixed import issue in sass 3.4.0

According to http://sass-lang.com/documentation/Sass/Script/Functions.html#index-instance_method
`index($list, $value)` returns `null` if `$value` is not found in `$list`.
Comparing it with `false` causes broken imports with middleman running sass 3.4.0. Comparing with `null` works here.
added it with `or` to stay compatible with older versions and libsass where `index()` seems to be returning `false` on mismatch.

Tested running middleman with sass 3.4.0 and grunt-sass with node-sass 0.9.3