Fix Tornado dependencies

Author: avara1986

Pipfile

@@ -12,7 +12,7 @@ anyconfig = ">=0.9.8"
 swagger-ui-bundle = ">=0.0.2"
 connexion = {extras = ["swagger-ui"],version = ">=2.2.0"}
 flask-opentracing = "==0.2.0"
-tornado = "==5.1.1"
+tornado = "==4.5.3"
 
 [dev-packages]
 requests-mock = "*"
@@ -24,3 +24,6 @@ tox = "*"
 
 [requires]
 python_version = "3.6"
+
+[pipenv]
+allow_prereleases = true