Move to a more recent ruby version.
Remove a duplication in the Gemfile.
Add `slug` column to events.
Index the slug for faster lookups.
Add validations for Event slug.
Add slug to seeded Event.
Replace references and lookups using `event_id` to use `event_slug` instead

It's kinda messy and reptitive in some places, but once we get Organization/User
management in, we'll probably see good patterns for extraction/refactoring.
Merge pull request #78 from jnf/jnf/words_not_numbers

Fixes #48. Use a slug to refer to event in paths instead of a numeric id.