Minor rewrite of slug generation duplicate check.

1. Uses "pluck" to pull only the slug column rather than instantiating
all objects.
2. Assumes that the slug ends with a serial number, finds the highest
such serial number, and adds 1 to it
3. This change should make acts_as_url usable for large datasets.
Merge pull request #165 from mdchaney/master

Minor rewrite of slug generation duplicate check.