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.