don't compile files if they haven't changed

package.json

@@ -15,8 +15,9 @@
     "fs-readdir-recursive": "^0.1.2",
     "gulp": "^3.9.0",
     "gulp-babel": "^5.3.0",
-    "gulp-cached": "^1.1.0",
+    "gulp-newer": "^1.0.0",
     "gulp-rename": "^1.2.2",
+    "gulp-util": "^3.0.7",
     "gulp-watch": "^4.3.5",
     "istanbul": "^0.3.5",
     "lodash": "^3.10.0",
@@ -29,6 +30,7 @@
     "rimraf": "^2.4.1",
     "semver": "^5.0.0",
     "shelljs": "^0.5.1",
+    "through2": "^2.0.0",
     "uglify-js": "^2.4.16"
   },
   "babel": {