Upgrade to Rails 4

QA Plan
-------

- [x] Deploy to iswhen-stage and make sure things work
- [x] Make sure tests pass
- [x] Resolve warnings from booting server
- [x] Resolve warnings from running tests

Known Issues / TODO
-------

- [ ] ~~figure out why event#show includes the current event in the big list~~ (converted to a separate issue: https://github.com/pbyrne/iswhen.info/issues/26)