Update Simple Form Config / Fix Sign-up Validation

This closes #70.

- Update the Simple Form config files for Simple Form 3.1 & Bootstrap 3 using the generator:
`rails generate simple_form:install --bootstrap`
- Use the default simple form validation error message.

Before:
![image](https://cloud.githubusercontent.com/assets/1070096/5481267/b27ab80a-8603-11e4-898b-3abdffed3ce8.png)


After:
![image](https://cloud.githubusercontent.com/assets/1070096/5481270/c96addce-8603-11e4-816e-6bebfa84c599.png)