Run twisted tests under py3 (#905)

diff --git a/.travis.yml b/.travis.yml
index 5ca11d7..cddbaa8 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -73,8 +73,8 @@
     env: TOXENV=py37-randomorder
 
   # Make sure we don't break Twisted or urllib3
-  - python: "2.7"
-    env: TOXENV=py27-twistedMaster
+  - python: "3.7"
+    env: TOXENV=py37-twistedMaster
   - python: "3.5"
     env: TOXENV=py35-urllib3Master