Update rollup requirement from ^0.67.0 to ^0.68.0

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/commits/v0.68.0)

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

package.json

@@ -50,7 +50,7 @@
     "mocha": "^5.2.0",
     "mocha-chrome": "^1.1.0",
     "node-sass": "^4.9.4",
-    "rollup": "^0.67.0",
+    "rollup": "^0.68.0",
     "rollup-plugin-babel": "^4.0.3",
     "rollup-plugin-commonjs": "^9.2.0",
     "rollup-plugin-eslint": "^5.0.0",