Update the Minitest version to ~> 5.0

Hello,

This is just a little pull request that let the users relying on `RDoc::TestCase` use a more up to date version of
Minitest. This would be very nice if this patch could be in 4.1.0. Actually Rails' documentation has an issue with chars such as `?z` (see https://github.com/rails/rails/issues/7172) and 4.0.1 doesn't fix this issue (see [this gist](https://gist.github.com/robin850/32368f4ebd11a8d6e238)) but 4.1.0.preview.3 does.

Then I guess if we don't want to introduce backward incompatibilities, we could allow versions between `~> 4.0` and `~> 5.0` and still rely on `MiniTest::Unit::TestCase` but the latter will throw a deprecation warning.

Let me know if you want me to add anything else to my patch.

Have a nice day.