Test & make sure relups can handle upgrading behaviours

As mentioned on [Line 42 of dispcount_serv](https://github.com/ferd/dispcount/blob/master/src/dispcount_serv.erl#L42) and the README file, Dispcount hasn't been tested with relups.

The issue here is that the dispatcher behaviour used by dispcount is implemented by piggy-back riding a gen_server instance. In the case of a relup, OTP navigates down supervision trees to figure out what module runs what code.

Right now the list of modules contains `[dispcount_serv, CallbackMod]`, but it is unclear if more than one module is supported, and if so, how the relup procedure will be invoked.

Testing, figuring out a good relup strategy and making sure it is properly documented should be done.