add test entries for tornado 4

Author: minrk

.travis.yml

@@ -27,7 +27,7 @@ before_install:
       fi
 
 install:
-    - pip install --pre .[test]
+    - pip install --pre .[test] $EXTRA_PIP
     - pip freeze
     - wget https://github.com/jgm/pandoc/releases/download/1.19.1/pandoc-1.19.1-1-amd64.deb && sudo dpkg -i pandoc-1.19.1-1-amd64.deb
 
@@ -68,7 +68,11 @@ matrix:
           dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
         - python: 3.6
           env: GROUP=docs
-        - python: 3.6  # extra test for older tornados
+        - python: 3.6
+          env:
+          - GROUP=python
+          - EXTRA_PIP="tornado<5"
+        - python: 2.7
           env:
           - GROUP=python
           - EXTRA_PIP="tornado<5"