Update rollup requirement from 1.14.2 to 1.21.2

Updates the requirements on [rollup](https://github.com/rollup/rollup) to permit the latest version.
- [Release notes](https://github.com/rollup/rollup/releases)
- [Changelog](https://github.com/rollup/rollup/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rollup/rollup/compare/v1.14.2...v1.21.2)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -54,7 +54,7 @@
     "mocha": "^6.2.0",
     "mocha-chrome": "^2.0.0",
     "node-sass": "^4.12.0",
-    "rollup": "1.14.2",
+    "rollup": "1.21.2",
     "rollup-plugin-babel": "^4.3.3",
     "rollup-plugin-commonjs": "^10.1.0",
     "rollup-plugin-eslint": "^7.0.0",