correct the SimpleReport initializer to match client call from Benchmark::Suite

The calling client requires the constructor takes two args
```ruby
      if @report
        @reports[file] = @report
        @report = nil
      else
        @reports[file] = SimpleReport.new(start, fin)
      end
```

https://github.com/yangchenyun/benchmark_suite/blob/master/lib/benchmark/suite.rb#L101