Merge pull request #4 from scurker/greenkeeper/rollup-0.51.1

chore(package): update rollup to version 0.51.1

Author: scurker

package.json

@@ -42,7 +42,7 @@
     "mocha": "^4.0.0",
     "nyc": "^11.2.1",
     "prettier": "^1.6.1",
-    "rollup": "^0.50.0",
+    "rollup": "^0.51.1",
     "rollup-plugin-babel": "^3.0.2"
   }
 }