chore(package): update rollup to version 2.3.1

Closes #32

Author: greenkeeper[bot]

package.json

@@ -36,7 +36,7 @@
     "expect": "^24.1.0",
     "mocha": "^6.0.0",
     "nyc": "^13.1.0",
-    "rollup": "^1.0.0",
+    "rollup": "^2.3.1",
     "rollup-plugin-babel": "^4.2.0",
     "rollup-plugin-filesize": "^6.0.0",
     "rollup-plugin-node-resolve": "^4.0.0",