Rollup merge of #46694 - kennytm:temporarily-use-old-travis-image, r=aidanhs

Temporarily use the old Travis image.

Use it until travis-ci/travis-ci#8891 is fixed.

(See also https://blog.travis-ci.com/2017-12-12-new-trusty-images-q4-launch)

r? @aidanhs

Author: kennytm

.travis.yml

@@ -1,6 +1,7 @@
 language: shell
 sudo: required
 dist: trusty
+group: deprecated-2017Q4
 services:
   - docker