negative time execution for 'default_middleware' benchmark

[sir-sigurd]

$ djangobench  --control stable/1.11.x --experiment master default_middleware
Running benchmarks: default_middleware
Control: Django 1.11.6.dev20170906232200 (in git branch stable/1.11.x)
Experiment: Django 2.0.dev20170915140012 (in git branch master)

Running 'default_middleware' benchmark ...
Min: 0.000026 -> -0.000037: -0.7010x faster
Avg: 0.000233 -> 0.000210: 1.1073x faster
Not significant
Stddev: 0.00126 -> 0.00148: 1.1770x larger (N = 50)