Prefer the use of public_send over eval

`eval` should be used really carefully.
Remove unused variable
Simplify validation method
Deprecated role?

`#{@@role_column}?` was returning the role itself.
With a `?` in the method name, I'm expecting to receive `true` or
`false`.
Clean up specs

These tests are dups based on the specs above. No need to test it
again.
Speed up the suite

Validations don't need to hit the database.
Merge pull request #2 from cfabianski/helioth-remove-eval

Prefer the use of public_send over eval