Update for Minitest 5.0.x

General updates for Minitest 5.0 compatibility and updates for implicit
subject testing.

* Update dependency to ~> 5.0
* MiniTest -> Minitest
* MiniTest::Unit::TestCase -> Minitest::Test
* Remove anonymous classes for testing the implicit subject
* Add tests for `.must` and `.wont`
* Add true cases for `must` and `wont` with implicit subject
Merge pull request #2 from elskwid/minitest-five

Update for Minitest 5.0.x