Temporarily use the old Travis image.

kennytm · steveklabnik · commit bc3e2aea66f3 · 2017-12-14T15:11:19.000-05:00
Use it until travis-ci/travis-ci#8891 is fixed.
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,7 @@
 language: shell
 sudo: required
 dist: trusty
+group: deprecated-2017Q4
 services:
   - docker
 
