Update rutgers.css

[jvcasillas]

Point to new RU branding for title slide logo (example: https://www.jvcasillas.com/ru_xaringan/slides/index.html#1)

To contribute to this repo, please make sure to:

• Sign the CLA (contributor license agreement). The CLA assistant will remind you below if you have not signed the agreement before.
• Add a news item to NEWS.md and your name to DESCRIPTION (by alphabetical order) unless you have already been listed there.
• Provide a URL to a live example (and even better, a couple of screenshots) if you are contributing a new theme.

Thank you!

[CLAassistant]

All committers have signed the CLA.

[yihui]

Thanks!