Fix Rails 3.2 uncompatible `find_by` method

**As a** Rails 3.2 developer 
**In order to** be able to use _Simple Token Authentication_ 
**I want** it to use the `find_by_email` Rails 3 method 

**As a** Rails 4 developer 
**In order to** be able to keep using _simple Token Authentication_ 
**I want** it to use the `find_by` Rails 4 method