fix #1531 (ValueError: StatsEntry.use_response_times_cache must be set to True)

[cyberw]

taken from taojy123@ce0e4d9)

[codecov]

Codecov Report

Merging #1534 into master will increase coverage by 0.82%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1534      +/-   ##
==========================================
+ Coverage   81.91%   82.73%   +0.82%     
==========================================
  Files          28       28              
  Lines        2560     2607      +47     
  Branches      388      408      +20     
==========================================
+ Hits         2097     2157      +60     
+ Misses        365      354      -11     
+ Partials       98       96       -2     

Impacted Files	Coverage Δ
locust/stats.py	89.06% <0.00%> (+0.24%)	⬆️
locust/web.py	93.50% <0.00%> (+1.26%)	⬆️
locust/runners.py	84.57% <0.00%> (+1.33%)	⬆️
locust/clients.py	95.09% <0.00%> (+4.90%)	⬆️

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