Default release is not detected properly

[josevalim]

Currently in the source code, there is a logic to detect the default release when none is specified. However, it does not work as expected. I have attempted a fix but as the code comments are also outdated, I have decided to open an issue.

The current code does sort the releases but sorts them in ascending order:

https://github.com/erlware/relx/blob/master/src/rlx_prv_release.erl#L131

The code comment says they are supposed to be sorted in descending order though:

https://github.com/erlware/relx/blob/master/src/rlx_prv_release.erl#L114

Besides, this code comment says we should abort when the release names are different:

https://github.com/erlware/relx/blob/master/src/rlx_prv_release.erl#L133-L136

But the code behaves differently.