MiniTest 5 compatibility

MiniTest 5 was released last week and as expected breaks WebMock:

```
NoMethodError: undefined method `stub_http_request' for #<#<Class:0x007fefa1760bb8>:0x007fefa2b75270>
    /Users/timkurvers/webmock/minitest/webmock_spec.rb:7:in `block (2 levels) in <top (required)>'
```

Relevant breaking changes:

* `MiniTest::Unit::TestCase` has been [renamed](https://github.com/seattlerb/minitest/blob/master/History.txt#L26) to `MiniTest::Test` 
* Assertions count is [now available](https://github.com/seattlerb/minitest/blob/master/lib/minitest/assertions.rb#L8) as an accessor on [Runnable](https://github.com/seattlerb/minitest/blob/master/lib/minitest.rb#L211)

This pull request restores compatibility and additionally ups the MiniTest requirement to 5.0.x.