MiniTest

### Why?

According to ruby-doc Test::Unit is no longer under active development 
````
If you are writing new test code, please use MiniTest instead of Test::Unit.
Test::Unit has been left in the standard library to support legacy test suites.
````
http://www.ruby-doc.org/stdlib-2.1.0/libdoc/test/unit/rdoc/Test/Unit.html