Only forward attr_accessible if needed

```attr_accessible``` has been deprecated in Rails 4 in favour of strong parameters, and extracted to a gem. Attempting to use it without this gem results in the following error:

```RuntimeError: `attr_accessible` is extracted out of Rails into a gem. Please use new recommended protection model for params(strong_parameters) or add `protected_attributes` to your Gemfile to use old one.```

This change fixes that by only calling attr_accessible in Rails versions < 4 or when the ```protected_attributes``` gem is loaded.