Conform to model generators for extra behavior

While re-running a bootstrapping script that uses generators, I noticed that I could safely re-run the defacto model generators from active record, but not the enum generators.

I found that there was a bit of custom behavior in place to mimic what `NamedBase` was already providing the `Migration` generator in active record, so I switched to `NamedBase` as a superclass.

```
Finished in 14.63 seconds
182 examples, 0 failures

Randomized with seed 16369

Coverage report generated for RSpec to /home/zbelzer/repos/power_enum/coverage. 469 / 471 LOC (99.58%) covered.
```