Fixes for AA 0.6.1

With AA 0.6.1 i get
```undefined local variable or method `max_csv_records' for #<Admin::MyModelController:0x007fe69b29a278>```

This is because the method was removed in this commit https://github.com/gregbell/active_admin/commit/a8ad4f989caec3a9ffbc820a949f309d2ce1a832
Using ```max_per_page``` should be the same, right?

Tests was green just because AA 0.6.0 was forced by ```Gemfile.lock```.
I think it's a good practice do not include it in gems, read here: http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile