Debian sid is now Python 3.7 (#4602)

diff --git a/.travis.yml b/.travis.yml
index a71e0c7..8fda670 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -77,9 +77,9 @@
         - python: 3.6
           services: docker
           env: TOXENV=py36 DOCKER=pyca/cryptography-runner-buster
-        - python: 3.6
+        - python: 3.7
           services: docker
-          env: TOXENV=py36 DOCKER=pyca/cryptography-runner-sid
+          env: TOXENV=py37 DOCKER=pyca/cryptography-runner-sid
         - python: 2.7
           services: docker
           env: TOXENV=py27 DOCKER=pyca/cryptography-runner-ubuntu-rolling