Upgrade Gulp SASS

Gulp was spitting out a lot of noise about stuff being out of date. The
cause seemed to be our outdated version of Gulp SASS. This commit
upgrades Gulp SASS to the latest version.

Changes:
---
https://github.com/dlmanning/gulp-sass/compare/v2.3.1...v3.1.0

This is a breaking change because one of the underlying libraries
includes a breaking change. But looking at their release notes, it
doesn’t seem to affect any SASS features that we’re using:

https://github.com/sass/libsass/releases/tag/3.4.0

package.json

@@ -31,7 +31,7 @@
     "gulp-concat": "2.6.0",
     "gulp-include": "2.1.0",
     "gulp-load-plugins": "1.1.0",
-    "gulp-sass": "2.2.0",
+    "gulp-sass": "3.1.0",
     "gulp-uglify": "1.5.1",
     "hogan": "1.0.2",
     "jquery": "1.11.2",