Update dependency rollup to v1.20.0

Author: renovate-bot

package-lock.json

@@ -39,7 +39,7 @@
     "npm-run-all": "4.1.5",
     "phantomjs-prebuilt": "2.1.16",
     "prettier": "1.18.2",
-    "rollup": "1.13.1",
+    "rollup": "1.20.0",
     "rollup-plugin-commonjs": "10.0.2",
     "rollup-plugin-json": "4.0.0",
     "rollup-plugin-node-resolve": "5.2.0",