Bump gulp-sass to 3.1.0
Bump gulp-sass to 3.1.0

Improves compatibility with newer versions of Node. Closes #1148.
randmanrjr/FoundationPress-randman,colin-marshall/FoundationPress,olefredrik/FoundationPress,bellatl/FoundationPress,olefredrik/FoundationPress,davenaylor/FoundationPress,colin-marshall/FoundationPress,davenaylor/FoundationPress,randmanrjr/FoundationPress-randman,linuxbastard/FoundationPress,linuxbastard/FoundationPress,bellatl/FoundationPress