Use Minitest::StatisticsReporter instead of Reporter

I was getting the following error on `master`:

```
$ be rake test                                                                                         
[Coveralls] Outside the Travis environment, not sending data.                                          
Run options: --seed 64029                                                                              
                                                                                                       
# Running:                                                                                             
                                                                                                       
......................................................                                                 
                                                                                                       
Finished in 0.130653s, 413.3085 runs/s, 543.4242 assertions/s.                                         
                                                                                                       
54 runs, 71 assertions, 0 failures, 0 errors, 0 skips                                                  
/.../guard-minitest/lib/guard/minitest/reporter.rb:10:in `report': undefined local variable or method `results' for #<Guard::Minitest::Reporter:0x007fc491f5f278> (NameError)       
        from /.../guard-minitest/.bundle/gems/minitest-5.0.4/lib/minitest.rb:
584:in `each'                                                                                          
        from /.../guard-minitest/.bundle/gems/minitest-5.0.4/lib/minitest.rb:
584:in `report'                                                                                        
        from /.../guard-minitest/.bundle/gems/minitest-5.0.4/lib/minitest.rb:
114:in `run'                                                                                           
        from /.../guard-minitest/.bundle/gems/minitest-5.0.4/lib/minitest.rb:
46:in `block in autorun'                                                                               
```

Fixed by inheriting from `StatisticsReporter` instead of `Reporter`. I'd be happy to add a spec for this, but wasn't sure how to best implement it.