Remove ruby 1.9.2, Use private_constant

By the end of the month the [support for `ruby 1.9.2` is stops](https://www.ruby-lang.org/en/news/2013/12/17/maintenance-of-1-8-7-and-1-9-2/) - this PR removes the support for `ruby 1.9.2`.

One big advantage of `ruby 1.9.3` is, that is supports `private_constant`. I think everything that is meant to be private should be private - for example the messages. To achieve this `add_offense` should always get the message and not collect it somehow via magic. So it is also more clear how the code is working and where the message is coming from.

What do you think?