runner: import werkzeug.middleware.proxy_fix.ProxyFix from its new location

from
https://werkzeug.palletsprojects.com/en/0.14.x/contrib/fixers/#werkzeug.contrib.fixers.ProxyFix
to
https://werkzeug.palletsprojects.com/en/1.0.x/middleware/proxy_fix/#werkzeug.middleware.proxy_fix.ProxyFix

the behavior seems the same, default config in both cases is to trust one
level of proxy.
3 jobs for master
Status Job ID Name Coverage
  External
failed SlapOS.Eggs.UnitTest-Master.Python2

failed SlapOS.Eggs.UnitTest-Master.Python3

failed SlapOS.Eggs.UnitTest-Master.Python3