Use the older Travis CI image

* The new one produces many warnings like:
  warning: Insecure world writable dir /usr/local/clang-5.0.0/bin in PATH, mode 040777
* Workaround from https://blog.travis-ci.com/2017-12-12-new-trusty-images-q4-launch

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61209 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.travis.yml

@@ -21,6 +21,9 @@ dist: trusty
 
 sudo: false
 
+# temporary workaround for https://github.com/travis-ci/travis-ci/issues/8892
+group: deprecated-2017Q4
+
 compiler:
   - gcc