Upgrade Rollup to v0.21

Author: lemmabit

package.json

@@ -24,7 +24,7 @@
   "homepage": "https://github.com/mcasimir/gulp-rollup",
   "dependencies": {
     "gulp-util": "^3.0.6",
-    "rollup": "^0.20.2",
+    "rollup": "^0.21.0",
     "through2": "^2.0.0"
   },
   "devDependencies": {