MiniTest -> Minitest for 5.x compatibility

Minitest 5.x defines the `MiniTest` constant so everything was
working. These changes just make the 5.x support explicit.
Update gemspec MiniTest -> Minitest

* Make 5.x support explicit in the gemspec
Update README

* MiniTest -> Minitest
* Add requirements section
Merge pull request #1 from elskwid/minitest5

Minitest 5 support