Remove version restriction on byebug.

Latest version of byebug is 3.5.1, so let’s not restrict byebug at all. The `byebug` method seems to be a fairly stable interface to byebug starting the debugger in code crossing versions.

The reason to restrict minitest is because of the plug-in interface; if the interface changes in an incompatible way in minitest 6 (not likely, but possible), then we don’t want the minitest interface to break. The single interface to byebug seems to be safe to trust for any future version.
Merge pull request #2 from halostatue/patch-2

Remove version restriction on byebug.