Update _functions.scss

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 on middleman with sass 3.4.0 and grunt-sass with node-sass 0.9.3