Omit Devise validatable plugin since User validates

Fixes duplicate error when already registered email is entered:

![screen shot 2014-08-01 at 8 10 24 pm](https://cloud.githubusercontent.com/assets/1108361/3786989/a37854ce-19f3-11e4-8384-d43954e473ff.png)

http://stackoverflow.com/questions/13897844/how-to-remove-duplicate-validation-errors/13897948#13897948