Cleanup rsvps_controller update method

Currently there are two issues listed on CodeClimate for `rsvps_controller`:

- Method update has a Cognitive Complexity of 17 (exceeds 5 allowed). 
- Method update has 36 lines of code (exceeds 25 allowed).

As we plan to add onto this method it should be cleaned-up a little.  (see #114)

https://codeclimate.com/github/codeRIT/hackathon-manager/app/controllers/rsvps_controller.rb