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.

slapos/runner/run.py

@@ -110,6 +110,9 @@ def run():
   serve(config)
 
 def serve(config):
+  try:
+    from werkzeug.middleware.proxy_fix import ProxyFix
+  except ImportError:
     from werkzeug.contrib.fixers import ProxyFix
   workdir = os.path.join(config.runner_workdir, 'project')
   software_link = os.path.join(config.runner_workdir, 'softwareLink')