Enforce tornado 4.x until the Tornado 5.0 support is fixed

Tornado 5.x starts it's own event loop, breaking other packages:
jupyter/notebook#3397
http://www.tornadoweb.org/en/stable/releases/v5.0.0.html#backwards-compatibility-notes

Author: vdboor

Makefile

@@ -1,7 +1,7 @@
 .PHONY: install clean test retest coverage docs
 
 install:
-	pip install -e .[docs,test,async]
+	pip install -e .[docs,test,async,tornado]
 	pip install bumpversion twine wheel
 
 lint:

setup.py

@@ -20,7 +20,7 @@
 ]
 
 tornado_require = [
-    'tornado>=4.0.2'
+    'tornado>=4.0.2,<5'
 ]
 
 async_require = []  # see below