fix(dependencies): update gulp-sass version

gulp-sass is dependant on node-sass and the current version (2.3.1) tries to download an old version of node-sass that apparently is not available on GitHub anymore. Changing the gulp-sass version to latest version (3.1.0) solves the problem.

lib/dependencies.json

@@ -31,7 +31,7 @@
   "gulp-sourcemaps": "^2.0.0-alpha",
   "gulp-less": "^3.1.0",
   "gulp-postcss": "6.1.1",
-  "gulp-sass": "^2.3.2",
+  "gulp-sass": "^3.1.0",
   "gulp-stylus": "^2.5.0",
   "gulp-typescript": "^3.1.4",
   "gulp-tslint": "^5.0.0",
@@ -53,4 +53,4 @@
   "typescript": ">=1.9.0-dev || ^2.0.0",
   "uglify-js": "^2.6.3",
   "vinyl-fs": "^2.4.3"
-}
+}