Update the Minitest version to ~> 5.0

[robin850]

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 rails/rails#7172) and 4.0.1 doesn't fix this issue (see this gist) 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.

[drbrain]

minitest 5 doesn't ship with ruby and rdoc can't have dependencies that exist outside of the latest ruby version so I cannot merge this patch.

The deprecation warning spew from 5 makes it very difficult to read the test output so depending on either version is unworkable for me.

To have minitest 5 imported into ruby someone needs to rewrite the test-unit compatibility layer, then this could be applied.