remove attr_accessible from DbLog

`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.

https://github.com/rails/protected_attributes


see:
https://github.com/rails/rails/blob/90f00142e6ff6a674f89c51a6784309b24b246bf/activemodel/lib/active_model/deprecated_mass_assignment_security.rb