Update to newer gulp version

package.json

@@ -58,7 +58,7 @@
     "eslint": "^1.5.1",
     "eslint-config-pebble": "^1.2.0",
     "eslint-plugin-standard": "^1.3.1",
-    "gulp": "^3.9.0",
+    "gulp": "^4",
     "gulp-autoprefixer": "^3.1.0",
     "gulp-htmlmin": "^1.3.0",
     "gulp-inline": "0.0.15",
@@ -78,12 +78,13 @@
     "mocha": "^2.3.4",
     "postcss": "^5.0.14",
     "require-from-string": "^1.1.0",
+    "sass": "^1.94.2",
     "sinon": "^1.17.3",
     "stringify": "^3.2.0",
     "through": "^2.3.8",
     "tosource": "^1.0.0",
     "vinyl-buffer": "^1.0.0",
-    "vinyl-source-stream": "^1.1.0",
+    "vinyl-source-stream": "^2.0.0",
     "watchify": "^3.7.0"
   },
   "dependencies": {}