Minitest 5 update

[Vít Ondruch]

For inspiration, there were used some interesting approaches to execute 
the Minitest 4.x testsuite against Minitest 5.x:

http://pkgs.fedoraproject.org/cgit/rubygem-idn.git/tree/rubygem-idn.spec
http://pkgs.fedoraproject.org/cgit/rubygem-raindrops.git/tree/rubygem-raindrops.spec

But still, please discuss this topic with upstream preferably, since in 
Ruby 2.2, there might not be shipped any testing framework:

https://bugs.ruby-lang.org/issues/9711



Vít